Calculate the amount of 8% acetic acid needed to combine with water to make 600 ml of 3.5% acetic acid.

if you need x ml of acid,

.08x + 0(600-x) = .035*600
x = 262.5

check:
.08*262.5 = 21
.035*600 = 21

so, if you mix 262.5ml of 8% acid with 337.5ml of water (0% acid), there is the same amount of acid in both mixtures