Hey!
For 45-45-90 triangles you use you use the formula L(on the square root of)2
L represents the legs.
So you plug in
3(on the square root of)2 times (the square root of)2
You multiply the square roots to get
3(on the square root of)4
You simplify the square root of 4 to 2
So 3 times 2 = 6
So 6 is the answer
Hope this helped!