so what you would do is set up your equations. your equations are 9+6x (type a) and 16x+4(type b). set them equal to each other since you want to know how many years it would take them to be the same height.
9+6x=4+16x
get your x values on one side
so it would be like this:
9+6x=4+16x subtract 6x from 16x and subtract 4 from 9
your equation will now be this:
10x=5
divide 10 on both sides and you get 1/2.
so both plants will grow the same height in 1/2 of a year
hope that helps!