Question

54+x=39+4x quanto da

Answer 1

54 - 39 = 4x - x

15 = 3x

15/3 = x

x = 5

Answer 2

$54+x=39+4x\to\\\\ x-4x=39-54\to\\\\ -3x=-15~(-1)\to\\\\ 3x=15\to \\\\ x= \dfrac{15}{3} \to\\\\ \boxed{x=5 }$