Question

If angle HFG is complementary to angle ACD, then?

Answer 1

I can help, but do you have an image I can see?

Answer 2

If you had an image then I would be able to give yoy an exact answer but your question means that:

Angle HFG + angle ACD = 90°
So to find either angle, you would need to subtract one of the angles above from 90°

so yeah complementary angles are basically angles that has a sum of 90°