'Suppose `f(x)=e^(a x)+e^(b x)`, where `a!=b`, and that `f''(x)-2f'(x)-15f(x)

"Suppose `f(x)=e^(a x)+e^(b x)`
, where `a!=b`
, and that `f""(x)-2f'(x)-15f(x)=0`
for all `x`
. Then the product `a b`
is equal to
25
(b) 9 (c) `-15`

(d) `-9`"