The 144 degrees is on a straight line, meaning the angle next to it inside the triangle and 144 are supplementary. They add up to 180 degrees.
Coincidentally, the other two angles plus the third, unlabeled angle will add up to 180 degrees as well, meaning that the the 48+x = 144 degrees (according the property of equality, which is if a=b and b=c, then a=c). Solve for x from there:
