When dividing by a fraction, it is actually a multiplication. All you need to do is flip the second fraction over. This is known as the reciprocal. In this example the fraction we want to flip is 7/12. Flipping it results in the fraction becoming 12/7.
We then multiply through the fraction so numerator times numerator and denominator times denominator (top x top and bottom x bottom). This gives us an unsimplifed answer of 36/56. These numbers both have a factor of 4 so can be divided through by 4 to get a simplified answer. 36 divided by 4 = 9 and 56/4 = 14 which shows that 3/8 divded by 7/12 = 9/14