If it is a right triangle, then it has a 90º angle.
As we know, the sum of the 3 angles in a triangle is equal to 180º. If we knoe the triangle has one angle that is 90º and the other that is 72º. Then what is the measure of the other acute angle (x):
![\begin{gathered} 180º=\text{ 90^^ba + 72^^ba + x} \\ 180º\text{ -90^^ba -72^^ba= x} \\ 18º=\text{ x} \end{gathered}]()
The measure of the other acute angle is 18º.