Keep in mind the slope is actually -0.5. Slope is rise over run (from left to right). It rises negative 1 or drops 1 and runs 2.
There are a number of ways you can find the y-intercept. Let's stick with the slope intercept form.
I plugged in the coordinates (6, 3) and used the slope -0.5 that we figured out already. If those coordinates are correct, then your graph is wrong.