Explanation:
for the 2 lines to be parallel the angles of crossing the third line must be equal.
and it is a given for 2 crossing lines, that the right top angle is equal to the left bottom angle, and the left top angle is equal to the right bottom angle.
therefore, the 2 indicated angles must be equal :
3x + 48 = 6x
48 = 3x
x = 16