You don't have a shown equation, but I could say the process.
Where does the line intersect on the y axis? That's the y intercept. Say it's at two: your Y = mx + b equation would look like this:
y = mx + 2
You have the y intercept, so now count. The slope is rise over run: So count up, then over. (unless it's negative, then down then over.) Now you have the slope. Say it's 1/2:
y = 1/2x + 2.
That would be your linear equation! Plug in from there.
Sorry if you already know this, I just want to help!
