If x+y=5, let y = 0 to find the x-intercept: x=5. The x-int. is at (5,0).
Let x = 0 to find the y-int: y=5. The y-int is (0,5).
Plot (5,0) and (0,5), and then draw a straight line thru both points.
If x+y=5, solving for y results in y = -1x + 5
Comparing this to y = mx + b, m=-1 and b= 5. Again, the y-int. is (0,5).