If st=x+3 and tu= 4x-6 and you want to find su you nedd to know that you want to know what s and u are to do that you need to solve for s and u in each of the equations. so it would be that s=x+3/t and u= 4x-6/t. then if you njedd to find su then you multiply across. but when you multiply the numerator you see that you can foil and at the bottom you multiply across and it would be t squared. when you foil the numerator you get 4x^2+12x-6x-18. then you simplify 12x-6x and your final answer would be 4x^2 + 6x -18/ t^2.