Question 500135
Let X be the measure of the first angle.
X+21 is the measure of the second angle.
X-15 is the measure of the third angle.
180=X+X+21+X-15
180=3X+6
174=3X
58=X
58+(58+21)+(58-15)=
58+79+43=
180
The measure of the first angle is 58.
The measure of the second angle is 79.
The measure of the third angle is 43.
They add up to 180 degrees.
Q.E.D.