A straight line is 180°. So you can do:
∠ACH + ∠ACB = 180° (you can do∠ACG or ∠ACB, doesn't matter)
This is because ∠ACH and ∠ACB make up a 180° angle(straight line)
∠ACH + ∠ACB = 180° Since you know ∠ACH, you can plug it in
115° + ∠ACB = 180° Subtract 115 on both sides to find ∠ACB
∠ACB = 65°
The ∠GBF should be equal to ∠ABC, so ∠ABC = 35°
This is because opposite angles are congruent/identical (equal)
A triangle is also 180°.
Now that you know the angle of C and B, you can do this:
65° + 35° + ∠A = 180°
100° + ∠A = 180° Subtract 100 on both sides
∠A = 80°
