Answer:
Number of t-shirts Trevor have are 16
If Trevor have 16 t-shirts, than Marissa will have 10 t-shirts.
Step-by-step explanation:
Let, the number of t-shirts Trevor has = x and number of t-shirts Marissa has = y.
Since, they both have total of 26 t-shirts. We get,

Moreover, Marissa has 6 t-shirts less than that of Trevor.
So, we have,

On solving the system of equations, we get,
implies
implies
i.e. 2x= 32 i.e. x= 16.
So,
implies
i.e y= 10.
Hence, the number of t-shirts Trevor and Marissa have are 16 and 10 respectively.
That is, If Trevor have 16 t-shirts, than Marissa will have 10 t-shirts.