1.) the length of the missing side is?

2.) what is the length of the missing side?

A^2 + b^2 = c^2
a^2 + 15^2 = 17^2
a^2 + 225 = 289
a^2 = 289 - 225
a^2 = 64
sqrt of 64 = 8
a = 8
x = 8

To find the hypotenuse the formula would be:

c^2 = a^2 + b^2

BUT! These two questions ask for the length of the smaller sides, so we need to rearrange the formula to:

a^2 = c^2 - b^2

Where c is the hypotenuse, and b is the other known length. Use this formula to solve the length of the unknown side. 

Let's call the unknown side x,

1) a^2 = c^2 - b^2
    x^2 = 17^2 - 15^2
    x^2 = 64
    Square root both sides to get x
    x = 8

2) a^2 = c^2 - b^2
    x^2 = 26^2 - 24^2
    x^2 = 100
    Square root both sides to get x
    x = 10

Hope this helped! Ask me if there's any part of the working you don't understand :)