15) Function notation:
f(x) = x² - 5x +8
16) f(2) , means you have to replace x by 2
f(2) = 2² - 5(2) + 8 = 4-10+8 and f(2) = 2
17) f(x) = 8 , means : 8 = x² - 5x +8
Simplify x² - 5x = 0
Factorize x² - 5x = 0 → x(x - 5) = 0, then the valueS of x are:
x = 0 and x = 5
