how do i find the equation of a line, that is on a grid?
recall, a line is of the form $\displaystyle y = mx + b$ where $\displaystyle m$ is the slope and $\displaystyle b$ is the y-intercept.
identify two points the line passes through, this is usually easy on a grid.
call one point $\displaystyle (x_1,y_1)$ and the other $\displaystyle (x_2,y_2)$
find the slope:
$\displaystyle m = \frac {y_2 - y_1}{x_2 - x_1}$
then plug that value for m into the point slope form:
$\displaystyle y - y_1 = m(x - x_1)$
solve for $\displaystyle y$ and you're done