algebra

Gather up like terms...

f^3 + 9f^2 -7f + 15

take f out as a factor in the first 3 terms

f (f^2 + 9f - 7) + 15

I don't think you can easily factorise the stuff in the () so this may be as far as it goes...

simplify f^3+f^2+17+8f^2-7f-2

= f^3 + f^2 + 8f^2 - 7f +17 -2

= f^3 + 9f^2 - 7f +15

just the same as Nod told you. But this is where you need to stop.

Simplify in this context means take out any brackets and collect like terms.
definitely do not put any brackets in when you simplify something like this.
If the aim of the question was to put brackets in, you would have been asked to factorise. (spelt with a z in USA I believe)