This is an isosceles triangle, that means that two of the sides are the same(it also means that two of the angles are the same). The red tick marks show which sides are the same, so ∠O and ∠M are the same.
A triangle is 180°, so the three angles add up to 180°. This means you can do:
∠O + ∠M + ∠N = 180
(5y - 18) + (5y - 18) + (8y) = 180
5y - 18 + 5y - 18 + 8y = 180 Combine like terms
18y - 36 = 180 Add 36 on both sides
18y = 216 Divide 18 on both sides
y = 12
Now that you know y, you can find ∠O:
∠O = 5y - 18
∠O = 5(12) - 18
∠O = 60 - 18
∠O = 42°
