From the given information, we can note that angle1 and angle2 are corresponding angles with respect to line u. Angle2 and angle9 are alternate exterior angles with respect to the right vertical line and angle5 and angle7 are alternate interior angles with respect to line v.
Therefore, the theorems which justify the answer are, respectively: Corresponding angles theorem, Alternater exterior angles theorem and Alternate Interior angles theorem.
Basically the question is asking YOU when you hear “system of equations” what do you think about it - what do you already know about system of equations. Basically. Btw enjoy this picture.
