Volume of sphere formula is:
V = 4/3 Pi r^3
It just is - you have to remember that.
We don't have radius, we have diameter. Radius is just half the diameter - so 7cm
V = 4/3 Pi (7)^3
V = 4/3 (3.14) (343)
V = 1436 cm^3 - remember its centimeters cubed because we are working with volume