f(x)=x*3 - 9x*2 + 15x + 2

Question

find function maximum

Answer 1

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