This is a system of equations problem, so we need two equations to represent each case:
5s+5L=155
10s+12L=346
Now we isolate one of the variables.
5s = 155 - 5L
s = 31 - L
Now we substitute s in the other equation with this equation.
10(31-L) + 12L = 346
310-10L + 12L = 346
2L = 36
L = $18
Now to find s we need to plug L into one of the equations.
10s + 12(18) = 346
10s + 216 = 346
10s = 130
s = $13
A large box costs$18 and a small box costs $13
Hope this helps.