Chapter 1.2, Problem 38E

Find all solutions $x_1,x_2,x_3$ of the equation $\stackrel{\to }{b}=x_1{\stackrel{\to }{v}}_1+x_2{\stackrel{\to }{v}}_2+x_3{\stackrel{\to }{v}}_3,$ where
$\stackrel{\to }{b}=\left[\begin{array}{c}-8\\ -1\\ 2\\ 15\end{array}\right],{\stackrel{\to }{v}}_1=\left[\begin{array}{c}1\\ 4\\ 7\\ 5\end{array}\right],{\stackrel{\to }{v}}_2=\left[\begin{array}{c}2\\ 5\\ 8\\ 3\end{array}\right],{\stackrel{\to }{v}}_3=\left[\begin{array}{c}4\\ 6\\ 9\\ 1\end{array}\right]$ .