This is pretty simple to do.
To start off call the number of days x, we can then formulate the following equation:
As we want the day where these two are equal to each other, we place the two equations equal to each other:
This rearranges to give us
Therefore after 18 days the plants will be the same height, on the 12th of July.
Hope this helps, just post back if there is anything you do not understand