Hi Nafis!
Here are two ways of thinking about it:
1) Imagine 3/8 as 3/8 of a pizza. Pizzas usually are broken into 8 slices, so 3/8 of a pizza is 3 slices. Similarly, 3/4 of a pizza is (think about it) 6 slices. So the problem is basically:
3 slices minus 6 slices
But you only have 3 slices to give! So if you try to minus more than you have to give, you owe slices. In this case, if you have 3 slices and you minus 6, you give all your slices away and still end up owing 3 slices. That's 3 slices. But slices are parts of a pizza so the answer is 3/8
2) 3/8  3/4 . Find a *common denominator* > in this case, 8. Convert the 3/4 into an equal fraction with an 8 in the denominator by multiplying both numerator and denominator by 2. Then the problem becomes:
3/8  6/8
Then, since 3  6 = 3, combine the numerators and you get:
3/8
I hope at least one of those was helpful to you! :)
1/9/2014

Murtaza N.