 
Arturo O. answered  11/25/19
Experienced Physics Teacher for Physics Tutoring
I assume you meant to say solve for x in f(x)=0. Use the quadratic formula.
x = [-b ± √(b2 - 4ac)] / (2a)
a = 1
b = -4
c = 5
Plug in the numbers, evaluate, and simplify. Note that the discriminant is negative, so you will get two complex roots.
 
     
             
                     
                    