from the information that it's an isosceles triangle. That means there are 2 (equal) sides - PR and PQ so the angles are also equal.
if you see that PQ and line Q are pararell so when there is a line like QR pass through them
the angle at R and at Q are the same ---- equal X. This time you know that R angle is X so Q angle is X too. because they are the side which has same length as I told you at first.
and in triangle, the total angle is 180° then
70+X+X = 180, 70+2X=180 SO 2X=110 ---> X= 55°
HOPE THIS CAN HELP :)
