7 2/5 minus 17/10
You have to put the whole thing over the same denominator (you have to think about the 7 as 7/1) (the denominator is the number on the bottom of the fraction)
So,
(7 x 10)/10 + ((2 x 2)/10) - (17/10) = (70 + 4 - 17)/10 = 57/10 = 5.7