The error is -6
Explanation:
If a line passes through the points
and

Then the equation of line is

Here given points are (-3,-7.5) and (2,-5)
Then the equation of the line is

⇔

⇔2.5(x+3) = 5(y+7.5)
⇔(x+3)= 2(y+7.5)
⇔x+ 3 = 2y +15
⇔2y = x+3-15
⇔y =

⇔y = 0.5 x -6
The error is -6