You just want to perform the opposite actions.
So in this case, it's multiplying with ?.
You want to isolate the question mark. So to do this, divide both sides by 4/7.
4/7(?) / 4/7 = 16 / 4/7
? = 16 / 4/7
Dividing by a fraction is basically the same as multiplying by the reciprocal, or the flipped around version, of the fraction.
16 / 4/7 = 16 * 7/4
16 * 7/4 = 28
? = 28