Which of the following cohld be the legnth of the sides of a 45° -45°-90°

I'm not exactly sure what "the following" means in your question. 
Normally, it means that there's more coming, like a list of choices,
but this time it must mean something else that I don't understand.

In a right triangle with equal acute angles, the lengths of the legs
are equal, and the length of the hypotenuse is 

                           (square root of 2) · (the length of either leg).

Okay our mixed number will be 7 and 1/4. STEP 1. Change it in to an improper fraction 7×4=28+1=29. The new fraction is 29/4. STEP 2. Divide the numerator (top number) by the denominator (bottom number). 29÷4= 7.25.