Answer:
3.5
Step-by-step explanation:
The angles in this triangle add up to 180 degrees, so the other angle is 45 degrees
you can either use the fact that it is a 45-45-90 triangle or some trig to solve; I prefer trig:
sin(theta) = opposite / hypotenuse
sin(45) = 2.5 / x
x = 2.5 / sin(45)
x = 3.5
if you want to use the 45-45-90 identity, you know that the other side is also going to be 2.5, and the hypotenuse will be the side length * square root of 2
x = 2.5 * square root (2)
x = 3.5
now to check your answer, use Pythagorean identity:
x^2 = a^2 + b^2
x = square root (a^2 + b^2)
x = square root (2.5^2 + 2.5^2)
x = 3.5
I hope it makes sense, leave a comment if I should go into more detail :)