f(x)=x*3 - 9x*2 + 15x + 2
find function maximum
usually ^ is used for exponents, * for products.
Assuming you meant
f(x)=x^3 - 9x^2 + 15x + 2
f(x) has max.min where f'(x) = 0
f'(x) = 3x^2 - 18x + 15
= 3(x-1)(x-5)
so, f'(x)=0 when x=1,5
f(1) = 9 a max
f(5) = -23 a min