Answer:
Part 1) The rate of change is
Part 2) The initial value is 68
Part 3) The function rule to the linear model is 
Step-by-step explanation:
we know that
The linear equation in slope intercept form is equal to

where
m is the slope or unit rate
b is the y-intercept or initial value
step 1
Find the slope
take two points from the table
(0,68) and (15,85)
The formula to calculate the slope between two points is equal to
substitute the values
In a linear function , the slope is the same that the rate of change
therefore
The rate of change is
step 2
Find the y-intercept
we know that
The y-intercept is the value of y when the value of x is equal to zero
Looking at the table
For x=0, y=68
therefore
The y-intercept is
The y-intercept is also called the initial value
therefore
The initial value is 68
step 3
Determine the function rule to the linear model

we have
substitute

3.0 NOT SURE IF ITS CORRECT
To find y, divide the first equation by 2.
y = 4
To find x, use that value for y in the second equation. Add 4, then divide by 2.
15·4 = 2x -4
60 +4 = 2x
64/2 = x = 32
The solution is (x, y) = (32, 4).
We set up a proportion: 80/?= 32/100
Cross multiply: 32*?= 80*100
⇒ ?= 80*100/32= 250
80 is 32% of 250.
I think you can use cosine rule if only the lines that form the <52 are equal.