1. The problem asks to find the measure of the missing angle $a$. 2. We know that the two angles $a$ and 51° form a right angle, which means their sum is 90°. 3. The formula for complementary angles is: $$a + 51 = 90$$ 4. To find $a$, subtract 51 from both sides: $$a = 90 - 51$$ 5. Calculate the difference: $$a = 39$$ 6. Therefore, the measure of the missing angle $a$ is 39°.