we know that
Average Rate of Change. is the change in the y-value divided by the change in the x-value for two distinct points on the graph
So

in this problem

substitute the values in the formula above

Divide by
both numerator and denominator

therefore
the answer is

x=11
x is equal to 11 because you subtract 4x from both sides and then add 8 to both sides where you are left with 2x=22. After dividing by 2, you get x=11
16 * 2 = 32
1 gal. = 2 half gal.
ans:32 days