Hey there!
So the first angle is a complementary angle, meaning that two angles put together will equal 90 degrees. Since we already know that one angle is 75 degrees, all we need to do is 90-75 and we get 15 degrees, which is the value of the missing angle.
The second angle is a supplementary angle, which means two angles equal 180 degrees. Since we know that one angle is 121 degrees, we can take that and subtract is from 180 degrees to get 59 degrees, which is the value of the missing angle.
Remember:
A complementary angle is made of up TWO angles that add up to 90 degrees.
A supplementary angle is made up of TWO angles that add up to 180 degrees.
I hope this helped!!!