We need to use the fact that the lines m and n are parallel (m||n). Thus we can identify the angles that congruent, that are equal.
In this case, we can see that the following angles are the same:
That is because they are on the same side in the parallel lines.
Next we use the fact that angles opposite to a vertex are equal:
And we apply this to our problem:
Thus we will have the following equation:
And we solve this