So, we see that the 90° is an exterior angle. since there's a straight line, this means that the 90° and the angle inside the triangle are supplementary angles, or they add up to 180 degrees, because the line doesn't change direction when it intersects. If the angle inside is supplementary to 90°, then this means that it is also 90°
Now let's look at the 125° exterior angle and use the same concept. 125 less than 180 is 55, so the interior angle is 55 degrees.
Now that we have two known angles, all we need is the angle measure of p. To do that, we first have to remember that all angles inside of a triangle also have a sum of 180°, so simply just add 55 and 90. After that, subtract it from 180.
So angle p is 35 degrees. I hope this helped. : )