First you have to find the y-intercept:
To do this, you can substitute the given values into a slope-intercept equation and solve for b.
y= mx + b
3= 4(-1/2) + b
3= 2 + b
b = 1
Now plug the values into an equation
y= mx + b
y = -1/2x + 1
Now graph the equation by placing a point on number 1 on the y-axis, then since the slope is -1/2 place another point 1 unit down and two units to the right. Connect the two points and you have your line.