Question

................................. 3х+21=39х

Answer 1

3x + 21 = 39x
3x - 39x = -21
-36x = -21
x = 7/12

Answer 2

$3x+21=39x \\ 21=39x-3x \\ 21=36x \\ x=\frac{21}{36}=\frac{7\cdot 3}{12 \cdot 3} = \frac{7}{12}$