What is the measurement of angle a ?

The interior angles of all triangle add to equal 180 degrees. This means you can set the measures of all these angles equal to 180 and solve algebraically for x.

    180 = (2x - 10) + (x + 30) + 70
    180 = 2x - 10 + x + 30 + 70
    180 = 3x + 90
    90 = 3x
    30 = x

To find m∠A, substitute 30 for x into 2x - 10.

    2x - 10
    2(30) - 10
    60 - 10
    50

m∠A = 50°

A= b/2 because u simplying both sides of equation