The wheel of the covered wagon turned around once in about 12 feet. the diameter of the wheel was a?

Well, the circumference of the wheel is π times the diameter. If the wheel turns once, the amount it's travelled is a circumference worth. So πd = 12. Divide both sides by π. And you get...

d=12/π

A very rough approximation of π is 3, so d = 12/3 (i.e. four feet).