2 can go into both of those numbers. So, divide 2 by both of the numbers.
42 / 2 = 21
56 / 2 = 28
So, make that a fraction: 21/28
7 can now go into both of those numbers. So, divide by both of the numbers.
21 / 7 = 3
28 / 7 = 4
Now, make that a fraction: 3/4
42/56 in simplest form is 3/4
I hope this helps!