Let T, S and P respectively the price of 1 T-shirt, 1 pair of shorts and 1 pair of pants.

So we have to solve a system with three linear equations to find T, S and P:

T + S = 16
T + P = 18
6T + 4S + P = 88

First find S and P in the first and second equations:

S = 16 - T
P = 18 - T

Then substitute them into the last equation and solve it to find T:

6T + 4(16 - T) + (18 - T) = 88
=> 6T + 64 - 4T + 18 - T = 88
=> 6T - 4T - T = 88 - 64 - 18
=> T = 6

With T we can find also S and P:

S = 16 - 6 = 10
P = 18 - 6 = 12

Then:

S + P = 10 + 12 = 22