Note that it said find "a" formula for P(x). We can infer from this as there are multiple possibilities.
In fact, there are many possible P(x) that fits the bill.
You got the most important part correct, namely
(x+5)(x-5)^2.
In general, P(x)=A(x+5)(x-5)^2 will work, but we need to satisfy also y-intercept of P(0)=-75.
If we expand the expression, we get
P(x)=A*x^3A-5A*x^2-25A*x+125A
=>
P(0)=125A.
So setting A=-3/5 => P(0)=-75 as required.
or
P(x)=-3/5(x+5)(x-5)^2
will satisfy all requirements.
It would be nice exercise for you to confirm that it is indeed the polynomial required.