Use substitution method to do it. Let S represents the number of swordfish and G to be number of goldfish. Since total number of fishes is 16, it can be written as S+G=16. Also they mentioned that the cost of a swordfish is $3 and the cost of goldfish is $5 and that the total cost is is $62, therefore 3s+5g=62. So, you can make one of the fish type as the subject (either s or g) and substitute this into the second equation.