F(x)=(x*x)-8x+15

Mmmm

There is no equal zero which is what you have all solve for.

In fact there is NO question here.

Perhaps we were supposedto factorise it in which case the answer is

$$\\F(x)=x^2-8x+15\\\\
\mbox{You need to find to numbers that mult to 15 and add to -8 (-3 and -5)}\\\\
F(x)=(x-3)(x-5)\\\\$$

And that is all that you can do.  

{x=4-sqrt(f(x)+1)

x=sqrt(f(x)+1)+4}

These are the two seperate answers you can recieve. 

{x=4-sqrt(f(x)+1), x=sqrt(f(x)+1)+4} ,good luck with that

f(x) = x^2 - 8x + 15

f(x) = (x - 5)(x - 3)

x - 5 = 0 --> x = 5.

x - 3 = 0 --> x = 3.

x = 3, 5.