1) Set up an equation for the total, with T representing Trevor and M representing Marissa:
T + M = 26
2) Use the provided information related Marissa's t-shirts to Trevor's to solve for one variable at a time (in this case, T):
M = T-6
3) Substitute M with this equation into the initial, first equation:
T + (T-6) = 26
4) Combine like terms and isolate T:
T+T = 26 + 6
2T = 32
T = 16
5) Therefore, you now know that Trevor has 16 t-shirts. To find the number of t-shirts that Marissa has, simply plug in this amount for T into the equation that was already set up for M:
M = T - 6
M = 16 - 6
M = 10
Therefore, Trevor has 16 t-shirts and Marissa has 10 t-shirts.
Hope this helps, and don't forget to mark my answer as the Brainliest! :)