a. $\left\{\begin{array}{l}5 x+2 y=9 \\ x+y=-3\end{array}\right.$

Step 1 ：Multiply the second equation by 5: \(\begin{cases} 5x + 2y = 9 \\ 5x + 5y = -15 \end{cases}\)

Step 2 ：Subtract the first equation from the modified second equation: \(3y = -24\)

Step 3 ：Divide by 3 to find y: \(y = -8\)

Step 4 ：Substitute y back into the first equation: \(5x + 2(-8) = 9\)

Step 5 ：Solve for x: \(x = 5\)

Step 6 ：\(\boxed{x = 5, y = -8}\)