Hi, so in order to graph you need to know what all those numbers mean.
The -1/2x is the slope or the rise/rub
The -3 is the y-intercept
So start by placing the y-intercept (0,-3), then from there place the slope (-1/2), since the slope is negative instead of going up one and two to the left you are going to go down one and two to the right, then place a point and repeat. After repeating 2-3 times you should be able to connect the points in order to make a line. If you have any questions let me know!