The answer to the question is D
I'm sure you know the formula for Pythagoras Theorem.
The formula is 
C = Your hypotenuse
A + B = The other two adjacent sides.
So you plug in the numbers

And you'll get the answer, which is 56.
Slope intercept form will help you find the function of the line :) [ y = mx + b ]
at time = 0 (y-intercept) the pool is 72in deep
the pool loses 2 inches of water every 1 hour. or -2/1
so your equation is y = -2x + 72
Good Luck!
X = 22/4
comment if u need explanation
If your trying to get Y=mx + b the answer would be 1. Y= x+4 2. Y= -1/3x + 4/3