Below, but added information.

So the measures of supplementary angles add up to 180 degrees. Knowing this, we can set up a system of equations:

 

First, let:

m∠T = t

m∠S = s

 

t = 3s

t + s = 180

 

Substitute 3s for t in the second equation:

3s + s = 180

4s = 180

s = 45

So m∠S = 45 degrees.

 

Now, plug 45 back into the first equation:

t = 3s

t = 3(45)

t = 135

 

So m∠T = 135 degrees. :)