I need to find the image of a line under a specific operator, but there are 4 variables, two of which are direct results of a transformation.

Suppose that T maps (x,y) into (s,t) is the linear operator on R2 defined by the equations:

2x+y=s

6x+2y=t

Find the image of the line x+y=1 under this operator.

Well, I know that the standard matrix is

2 1

6 2

but how do I find the image of x+y=1?