(a) Perform the indicated row operations on the matrix $A$ successively in the order they are given until a matrix in row echelon form is produced. $$A = \left[\begin{array}{ccc} 5 &-15 &5\cr 7 &-20 &4 \end{array}\right]$$
Apply $(1/5) R_1 \to R_1$ to $A$.
Apply $R_2 - 7 R_1 \to R_2$ to the previous result.

(b) Solve the system $$\left\lbrace \begin{array}{rcl} 5 x_1 - 15 x_2 & = & 5 \\ 7 x_1 - 20 x_2 & = & 4 \\ \end{array} \right.$$ $\vec{x} =$ $\left[\Rule{0pt}{2.4em}{0pt}\right.$$\left]\Rule{0pt}{2.4em}{0pt}\right.$