By the , the sum of the angles in a triangle is equal to 180°. therefore, m? a+m? b+m? c=180° . using the , (5x)°+90°+(x+10)°=180° . to solve for x, first combine like terms to get 6x + 100 = 180. using the , 6x = 80. then, using the division property of equality, x = 1313 . to find the measure of angle a, use the substitution property to get m? a=5(13113)° . finally, simplifying the expression gets m? a=6623° .