When you add up the degrees of the corners of the triangle, your total should always be 180°. At first look x seems to be part of a very small triangle with no known angles. Look beyond at the larger triangle x is a part of and you have the angles for both other corners. Simple calculations put the sum of 42° and 73° at 115°. Since a triangle must always add up to 180°, the difference between what you have and what you should have will be your solution for x.
In short, 180-115=65. X is 65°.