( assume that 3y - t # 0.)
t(t-x) = 3y(3y-x)
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.