f(1/2)=(1/2*1/2)-2(1/2)+3

I assume you have a function that's probably fx) = x² - 2x + 3 and you have to evaluate it at x = 1/2.  In which case you have written it down ok, so you just need to complete it:

(1/2)*(1/2) - 2*(1/2) + 3 = 1/4 -1 +3 = 1/4 + 2 =  $$2\frac{1}{4}$$