Let
x-------> the time in minutes
y-----> the function that represent the amount of water in gallons
Step 1
Find the x-intercept
we know that
The x-intercept is the value of x when the value of the function is equal to zero
In this problem (see the graph)
For

the value of x is

so
the x-intercept is the point

that means
The number of minutes it takes for the container to empty is

Step 2
Find the y-intercept
we know that
The y-intercept is the value of the function y when the value of x is equal to zero
In this problem (see the graph)
For

the value of the function y is

so
the y-intercept is the point

that means
The number of gallons in the container before the drain is open is

therefore
the answer in the attached figure