When two lines cross each other, four angles are formed.
An angle is equal to an angle that is opposite to it. In the above image ∠a is opposite (facing) ∠c, so ∠a = ∠c.
∠b is opposite ∠d so
m ∠b = m ∠d
Furthermore, when two lines cross each other, any two angles next to each other are supplementary.