you has to make the botom number the same
to change the bottom number and maintain the same value of the fraction you has to multiply by 1 or x/x wher x=x
so
bottom numbers are 7 and 3
make them 21 by mutliply 5/7 by 3/3 and multiply -1/3 by 7/7
5/7 times 3/3=15/21
-1/3 times 7/7=-7/21
now we can do
15/21-7/21=(15-7)/21=8/21
answer is 8/21