Question

What is the equation of the trend line?

Answer 1

So we use the form
$y = mx + b$
Find b by looking at the start point, in this case: 110. So we have:
$y = mx + 110$
Then, we see that it is a decreasing line, so we know that m < 0.
Filling in
$x = 20$
Gives y = 70, so solve the (function) equation:
$f(20) = 70$
$20 \times m + 110 = 70$
$20m + 110 = 70$
$20m = - 40$
$m = - 2$
Therefore the formula is:
$n = - 2m + 110$
Or:
N = -2M+110 as I couldn't find capital letters :-(