Two lines intersect forming four angles

Question

Two lines intersect forming four angles. If one of them is right, show that others are too straight.

I am clueless how to start. Ideas?

Answer 1

When two lines intersect, you have that opposite angles are equal, therefore if one is a right angle then the opposite is right too. We also know that two adjacent angles (of this intersection) add up to $180$ degrees, so if one is right, its adjacent one should also be right as to add up to $180$. Finally, the opposite of this one is equally right.

Answer 2

If 2 lines intersect at a right angle, then they are perpendicular. Now, perpendicular lines have all 4 angles being right. So by definition of straight angle, they are straight too.