Answer:
a = 5
Step-by-step explanation:
(x+3)(x+a)(x+7)=x³+15x²+71x+105
(x+3)(x+a)(x+7) = (x²+10x+21)(x+a) = x³ + (10+a)x² + (21+10a)x + 21a
21a = 105
check: 21 + 10a = 71 ... 71x
10 + q = 15 .... 15x²
