53°
Angle X + Angle 2x + 21 = 180. [ LINEAR PAIR ]
Angle 3x + 21 = 180
Angle 3x = 180 - 21 [ Transposing 21 over to RHS ] ( addition becomes subtraction)
Angle 3x = 159
Angle x = 159 ÷ 3. [ Transposing 3 over to RHS ] ( Multiplication becomes divison)
Angle x = 53°