A few problems:
1) you solved for the radius, not the diameter
2) although your formula for the radius is correct, the number you get at the end is not right. Are you sure that you are dividing by pi and not multiplying by it (for example if you type: 3/4π into a calculator, it will interpret this as 3 /4 * π = 3 * π / 4. Typing 3/(4π) gives you the result you need.)
(NB: if you are unsure if you rearranged a complicated expression correctly, you can always try plugging the answer back into the original equation. So if you try to calculate the volume with a radius of 7.026, you get:
V = (4/3)π*(7.026cm)^3 = 1453cm3
which is obviously not the right volume)