rearrange the following equation to find x in terms of y and t
y(y-x)=3t(3t+x)
multiply out the parenthesis.
gather the x terms on one side.
-yx-3tx=-y^2 + 9t^2
-x(y+3t)= -y^2+9t^2
now, divide both sides by -(y+3t)
x= (y^2-9t^2)/(y+3t)
the numerator is a difference of two squares, so will factor. One of those factors will divide with the denominator.