Hi there there's several ways this could be proven one way us to consider the allied angle theory where two angles formed between parallel lines are supplementary which in this case can be proven by
2(45)+90=180⁰ ✔
or 3(45)+45=180⁰✔
this would not be the case if it wasn't parallel
Consequently, you can also use the alternate angle theory where you essentially extend one of the lines and you'll see two equal alternate angles
