To Continue my post above:

You should draw all this out as a point in the middle of square surrounded by a circle. The circle is touching the corners of the square.

Area of a circle equation is (pi)*radius^2

Side of the square from the garden is square root of 920. That number is 30 since this is all whole number work.
To find the radius of the circle, we can assume since this figure is a square garden that if two intersecting lines went through the center the angles formed at the center would be 90 degrees(right angles) to each other. This forms 4 triangles in the square. You can the use the "sine" functino for this problem.
In a right triangle, there is a 90* angle, and two 45* angles. Match these up. Sine is opposite side over the hypotenuse.
The longest side has the greatest angle.

So its' sin(45*) = X/30.
.707 = X/30
X = (.707)(30) = 22

So the radius equals 22 and that the shortest radius one can have to water the whole garden.

TO Find the area that is not being watered but is just extra, it's the area of the circle MINUS the area of the square garden.

Area of Circle = (pi)(radius^2)
Area of Square = side^2

so it's (3.14)(21^2) - 30^2 = 485 square feet not being watered

There you go my man. It's not about derivatives at this level, use the easy math for easy problems.

If it was a harder problem, then use the calculus.

Dave