This question is about addition and subtraction
"our Eagles ran for 9 yards on the 1st down" means they moved forward by 9 yards = 9
"On the 2nd down, they lost 7 yards" means they moved backward by 7 yards = -7
"On the 3rd down, they ran for 35 yards" means they moved forward by 35 yards = 35
Total gain = distance covered (1st down + 2nd down + 3rd down)
Total gain = 9 - 7 + 35 = 37 yards