案例环境不起作用

Question

我正在尝试写这个：

$$\begin{cases}
    2a - b = 3 \\
    a + 2b = 4 \\
    3c - d = 1 + i \\
    c - 3d = -i
    \end{cases} \Longrightarrow 
    \begin{cases}
    2a - b = 3 \\
    a = 4 - 2b \\
    3c - d = 1 + i \\
    c = -i + 3d
    \end{cases} \Longrightarrow
    \begin{cases}
    2(4 - 2b) - b = 3 \\
    a = 4 - 2b \\
    3(-i + 3d) - d = 1 + i \\
    c = -i + 3d
    \end{cases} \Longrightarrow $$

但我不知道哪里出了问题。

\documentclass[a4paper,12pt,oneside,brazil]{icmc}

\usepackage{amsmath}         
\usepackage{color}    
\usepackage{xcolor}        
\usepackage{colortbl}        
\usepackage[top=25mm,bottom=25mm,left=25mm,right=20mm]{geometry} %coloca o tamanho das margens              
\usepackage[latin1]{inputenc}             
\usepackage[T1]{fontenc}            
\usepackage{setspace} %espacamento entre linhas               
\usepackage{natbib} %referencias com numera\c{c}\~{a}o              
%\usepackage[brazil]{babel}                   
\usepackage[portuges]{babel}                       
%\let\cite=\citep                 
\usepackage{url}                      
\usepackage{fancybox}                       
\usepackage{indentfirst} %indenta a primeira linha               
\usepackage{times}       %utiliza a fonte Times New Roman                  
\usepackage{graphicx} %pacote para incluir imagens                  
\usepackage{titlesec} %para os gr\'{a}ficos dos cap\'{\i}tulos            
\usepackage{textfit}                   
\usepackage{fancyhdr} % Para colocar cabe\c{c}alho nas p\'{a}ginas       
\usepackage{lettrine} % Para fazer letras capitulares         
\usepackage{hyperref}           
\usepackage{amsthm}          
\usepackage{amssymb}          
\usepackage{amsfonts}            
\usepackage{bezier}            
\usepackage{amstext}         
\usepackage{graphicx}          
\usepackage{subfigure}      
\usepackage{amscd}           
\usepackage{amsxtra}            
\usepackage{array}              
\usepackage{float}           
\usepackage{maple2e}           
\usepackage{pdfpages}            
\usepackage{mathtools}        

\begin{equation*}
            \begin{pmatrix}
            i & 1 \\
            -1 & i
            \end{pmatrix} \cdot \begin{pmatrix*}
            x \\ y
            \end{pmatrix*} = \begin{pmatrix}
            0 \\ 0
            \end{pmatrix} \Leftrightarrow  \begin{cases}
            i x + y = 0 \\
            -x + i y = 0
            \end{cases} \Leftrightarrow \begin{cases}
            y = -ix \\
            0 = 0
            \end{cases}.
            \end{equation*}