Answers:
x = 7
The measure of angle D is 14 degrees
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Angle C and angle D are complementary, which means they add to 90 degrees.
(angle C) + (angle D) = 90
(12x-8) + (3x-7) = 90
12x-8 + 3x-7 = 90
(12x+3x) + (-8-7) = 90
15x-15 = 90
15x-15+15 = 90+15
15x = 105
15x/15 = 105/15
x = 7
So you have the correct x value. Nice work.
Now use this x value to find the measure of angle D
angle D = 3*x - 7
angle D = 3*7 - 7
angle D = 21 - 7
angle D = 14 degrees
