Question

Six years after a tree was planted, its height was 7 feet. Nine years after it was planted, its height was 16 feet. Which of the following equations gives the height y, in feet, of the tree after x years if the tree grows at a constant rate? Check all of the boxes that apply.

Answer 1

Considering that the grows at a constant rate we can form an equation where x = how many years after it was planted
and y = its height

Now we just need to find how many feet it grows each year. To do that we just need to compare its height from a certain age to another:
6 years after it was planted : 7 feet,
so x=6 and y = 7

9 years after it was planted: 16 feet
so x= 9 y=16

With thay we can conclude that in 3 years , the tree grew 9 feet. To discover how much the tree grow each year we just nee to divide 9 feet by 3 years which is 3 feet every year.

To write the equatopn now we just need to find the y-intercept which we can discover by setting x to 0:
If in 6 years after the tree was planted it is 7 feet long , we can discover how long it was when it was planted by subtracting 6 years of growth (The slope ) which is 3
7 - 6(years)×3(feet the tree grow each year)
7 - 18 = -11
The tree was -11 feet long when it was planted
which is our y-intercept
( I know it doesnt make sense , but if you apply to a graph it will make more sense )


Now we can make the equation
y = 3x -11