Hi,
First let's find the unknown degree in the triangle. We know an entire triangle is 180 degrees so all we have to do is add the sum of the other two interior angles. 55 + 72 = 127. Now let's subtract 127 from 180 to get 53. Now let's solve for x. Since we know the angle beside it is 53, we subtract that from 180 degrees (a line segment is 180 degrees) to get 127.
x = 127 degrees
Have a great day!