Ok, I don't know if you're supposed to do it with fractions or with decimals. If with decimals, well, 3/4=0.75 (just divide 3/4 and you'll see) and 1 3/4 = 1.75
Then you divide 120/1.75 and there you go, 68.57mph
If you're supposed to do it with fractions:
1 3/4 = 4/4 + 3/4 = 7/4
Then you divide 120 / 7/4 (remember 120 = 120/1
Which means you have to cross multiply (an important part on fraction division, that is, your division would be 120x4 / 1x7).
Then 120x4= 480
And 1x7 = 7
Then you get 480/7, which is the same as 68 4/7 mph
I'm adding an example of a different fraction division.
I hope this helps a bit.