So you need to know your vocabulary. "Of" means to multiply. Now the "what" part can be a variable, since you don't know it. It can be x. So it would look like this:
3/4x=75. Now solve for x. Divide 3/4 on both sides and you get 3x=300. Now solve for x again. So divide 3 on both sides and you get 100. So the final answer would be x=100.
Sources: my brain
Hope this helps! :-)