To see if it is possible, you will want to create an equation.
3(4x+2) = 6(2x+2)
As you can see, you will need to use the distributive property for this equation.
12x + 6 = 12x + 12
Remove 6 from both sides.
12x = 12x + 6
Divide 12 by both sides
x = x + 0.5
Because the answer wasn't x = x, the two plants won't ever be the same height. Even if you tried to subtract 0.5, you'd only get a negative 0.5 on the other side.