Woo. Fractions divided by fractions are a doozy. For dividing fractions, I use the Keep, Change, Flip method. So we will keep 3/4, change the division sign to a multiplication sign, and flip the 6/7 to a 7/6. So we get this expression.
3/4 x 7/6
3 x 7 = 21, and 4 x 6 = 24. 21/24. The fraction is in simplest form.