Answer:
Should be 39 years.
there is 12 inches in a foot. and if the tree grows 4 inches every year, then in 3 years the tree would be 1 foot. 13 x 3 = 39, thus proving that in 39 years, it will be 13 feet tall. I may be wrong but this sounds correct.
So we have 2 variables here: tacos and orders of nachos.
When we translate the paragraphs into equation:

Now, in this situation we can make use the elimination method by converting 3n to -27n.

Add both equations:

So we find that one taco costs $2.75.
We can plug this into any of the first two equations to find n:

So one order of nachos cost $1.40.