First, you must turn it into slope-intercept form (y=mx+b)
12x+28y=84
-12x -12x
28y=84-12x
÷28 ÷28
y= 2/5x+3
the +3 is your y-intercept (the point which your line touches the y-intercept
then the 2/5x is the slope (slope: rise over run)
so starting from your y intercept go up 2 to the right 5, then if you need to go down from the y-intercept do the opposite (down 2 to the left 5) now just draw the line to connect the points
hope this helps!!