Hey there! I'm not sure what method your teacher would be looking for but my first thought would be using the Pythagorean theorem!
a^2 + b^2 = c^2
Where a and b are the legs while c is the hypotenuse
In this case the diagonal would be the hypotenuse so you could plug that in for c and the width is one of the legs so you can plug that in too.
a^2 + 15^2 = 25^2
Then
a^2 +225 = 625
Then
a^2 = 400
Then
a = 20
Now you know the width and length (15 and 20). So you can multiply both of them to get the area. So you answer is 300!!!
I hope that helps! Feel free to let me know if you have any questions!
- mathwizzard3