Question

sendo f(x)=2x+5 obtenha o valor de f(x)=21 

Answer 1

f(x) = 21
2x + 5 = 21
2x = 21 - 5
2x = 16
x = 8

Answer 2

 f(x)=2x+5        f(x)=21 
21 = 2x + 5
21 - 5 = 2x
16 : 2 = x 
x=8