well, we know the tree is growing at a rate of 36 feet every 3 years, how many feet is that in 1 year alone? well, simply 36/3 = 12.
so the tree is growing 12 feet per year.
when it was first planted, it was 5 feet tall, and then after every year, we have to add 12 feet subsequently, let's do that
year 1 .......... 5 + 12(1)
year 2 .......... 5 + 12(2)
year 3 .......... 5 + 12(3)
year 4 .......... 5 + 12(4)
year 5 .......... 5 + 12(5)
year 6 .......... 5 + 12(6)
year 7 .......... 5 + 12(7)
year y .......... 5 + 12(y)
[tex]\bf \stackrel{\textit{height}}{h}~~=~~\stackrel{\textit{initial height}}{5}~~+~~\stackrel{\textit{yearly rate}}{12y} \\\\[-0.35em] ~\dotfill\\\\ \stackrel{\textit{tree is now}}{77} = 5 + 12y\implies 72=12y\implies \cfrac{72}{12}=y\implies \boxed{6=y}[/tex]
