The general equation of a line is y = mx + b
Where m is the slope of the line and b is the y-intercept
We know b = 1500. Which also implies that the point (0,b) belongs to the line.
To calculate m, we use the two known points:
m = slope = rise / run = (2100 - 1500) / (10 - 0) = 600 / 10 = 60
Now the equation requested is y = 60x + 1500.
Use x = 3 to get the approximate sales for the third year:
y = $3*60 + $1500 = $180 + $1500 = $1680.