You would take 2/7 and multiply both the top and bottom by 2 to make it 4/14, the. youd flip 4/14 into 14/4, then multiply the top numbers (9 and 14 by eachother to get 126) then multiply the bottom numbers ( 14 and 4 to get 56) and simplify the new fraction (126/56) to get 9/4!
