Use Pythagoras’ theorem:
a^2+b^2=c^2
But we rearrange this formula to make it
c^2-a^2=b^2
So 25^2-15^2=b^2
625-225= b^2 = 400
You now need to find the square root of 400 to find the square root of b
sqrt of 400 is 20
So 20 is the missing length in this triangle
