For questions like this it is best to draw a diagram. Below I have drawn two triangles. One labeled ABC and the other PQR. As you can see the order that the letters are labeled are the same order as what they are here:
ABC and PQR
B and Q are in the same spot in the order
ABC and PQR
This means that angle B corresponds with angle Q
Hope this made sense! Let me know if you understand.
~Just a girl in love with Shawn Mendes