The roots of f(x) are {0, 3, -4}. You've got them as {-3, 4}, which is not correct.
Draw another set of coordinate axes and place dark dots at (0,0), (3,0) and (-4,0). These dots represent the roots (solutions) of the given polynomial. Note that we have a repeated (double) root at x=3, which is given away by the exponent 2 of (x-3).
A basic way of sketching this graph is described as follows:
Evaluate the function (find y) for several x-values other than (0, 3 and -4): Choose (for example) {-5, -2, -1, 1, 2, 4}
If you'll find the y-value for each of these x-values and plot the resulting points, you should see the shape of the graph. Draw a rough graph thru these points. If any doubt remains about what the graph looks like at particular x-values, calculate and plot more points, e. g., at {-2.5, -1.5, ...}.
If you're taking calculus, consider applying the First- and Second-Derivative tests to determine concavity, maximum, minimum, etc.
Measure the height and radius of the tank. The radius is the distance from the center of the tank to its outer edge. Another way to find the radius is to divide the diameter, or width, by two. Square the radius by multiplying the radius times itself and then multiply it by 3.1416, which is the constant pi.
Given height and volume: r = √(V / (π * h)),
Given height and lateral area: r = A_l / (2 * π * h),
Given height and total area: r = (√(h² + 2 * A / π) - h) / 2,
Given height and diagonal: r = √(h² + d²) / 2,
Given height and surface-area-to-volume ratio: r = 2 * h / (h * SA:V - 2),
Given volume and lateral area: r = 2 * V / A_l,
Given base area: r = √(A_b / (2 * π)),
Given lateral area and total area: r = √((A - A_l) / (2 * π)).
