In cases like this, where you want to express a variable, or a group of variables, in terms of the other variables, you just isolate the said variable, or the said group of variables.

For your problem here, just isolate the x.

t(t -x) = 3y(3y -x)

t^2 -tx = 9y^2 -3xy

Collect the x's in one side of the equation,

-tx +3xy = 9y^2 -t^2

x(-t +3y) = (3y -t)(3y +t)

x = 3y +t -----------------answer.