To find the slope we divide the difference between the y coords by the difference between the x coords:
(x²-3-13)/(x+1-5)=(x²-16)/(x-4)=(x+4)(x-4)/(x-4)=x+4. (The slope is continuous except for x=4 because at this point, the calculation of the slope would result in 0/0, which is theoretically indeterminate.)