For this problem, make a system of equations.
15s+8t=243 | 12s+6t=192
Then multiply the second equation by 4/3 so you can have a common monomial to add/subtract out. You now have:
15s+8t=243 | 16s+8t=256
After subtracting, you have:
s=13
Now you can substitute s into one of the equations.
12(13)+6t=192
6t+156=192
6t=36
t=6
The final answer is that a pound of salmon is $13 and a pound of tuna is $6.