I'm not sure how to solve this? Tanna purchases a rectangle mirror with a diagonal measure of 25 centimeters and a height of 15 centimeters. What is the perimeter of the mirror?

Use trigonometry! a^2+b^2=c^2, with a and b being the sides of the triangle/rectangle (depending on how you draw it) and c being the diagonal.

Your equation should be 15^2+b^2=25^2

b should equal 20

so 20+20+15+15=P=70