Question 4460
Start with the formula for the volume of a rectangular prism (this is the shape of the aquarium):  V = L*W*H  (That's Length * Width * Height)

L = 21 inches
H = W + 3

V = (21)(W)(W+3) = 2730 cubic inches.
    (21)(W^2 + 3W) = 2730 Cubic inches.  Divide both sides by 21.
    W^2 + 3W = 130   Subtract 130 from both sides.
     W^2 + 3W - 130 = 0  Solve the quadratic by factoring.
    (W + 13)(W - 10) = 0   Apply the zero products principle.

W + 13 = 0, W = -13 Discard this solution as negative width is not meaningful.
W - 10 = 0,  W = 10  The width of the aquarium is 10 inches.
H = W + 3
H = 13 The height of the aquarium is 13 inches.

Check:  Volume, V = (21")(10")(13") = 2730 cubic inches.