We know, the sum of the measures of angles in a triangle is 180°.
Therefore we have the equation:
x + (x - 8) + (3x + 13) = 180
(x + x + 3x) + (-8 + 13) = 180 combine like terms
5x + 5 = 180 subtract 5 from both sides
5x = 175 divide both sides by 5
x = 35
m∠A = (3x + 13)°. Substitute:
m∠A = (3(35) + 13)° = (105 + 13)° = 118°