Justin is skeet shooting. The height of the skeet is modelled by the equation h = -3t^2+16t, where h represents the height in feet t seconds after the skeet is
released. The path of Justin's bullet is modelled by the equation h = 7t, with
the same units.
How long will it take for the bullet to hit the skeet?
seconds
How high off the ground will the skeet be when it is hit?

I'd like to offer the alternative perspective. I think that learning grammar rules through a written account of the system (text books) is something that is only useful to a language learner under the following conditions:
they have already learned how to have a conversation in the language 
they are naturally curious now about how the patterns in how it fits together 
they would like to learn how to read in the language
they would like to improve their accuracy for a specific purpose (eg professional or educational)