Question

zero da funçao f (x) = x2 - 4x

Answer 1

x² - 4x = 0    
colocar x em evidência

X ( x - 4 ) = 0

x' = 0    x - 4 = 0
           x" = 4 

Answer 2

$f (x) = x^2 - 4x\\\\\\ x(x-4)=0\\\\\\ x=0\\\\ x-4=0\to x=4\\\\\\\\ S= \left \{ 0;4 \left \}$