5. $\frac{3}{2}=\frac{a - 3}{a + 1}$

Explanation:

Step1: Cross - multiply

$3(a + 1)=2(a - 3)$

Step2: Expand both sides

$3a+3 = 2a-6$

Step3: Move terms with $a$ to one side

$3a - 2a=-6 - 3$

Step4: Simplify

$a=-9$

Answer:

$a = - 9$