You deposited $500 in the bank at 6%, how would you use the Rule of 72 to find out how many years the money has to stay in the bank to be doubled? What about if you do not know the interest, but the numbers of years. Can you find the interest rate?

for finding the midpoint of any given 2 pair of points simply add the respective x coordinates with other x coordinate and the y coordinate with other y coordinate and divide them by 2

in your question it will be :

(3+1)/2 for the x coordinate of mid point i.e 2

[8+(-4)]/2 for the y coordinate of midpoint i.e 2

so coordinates of mid point of points (3,8) and (1,-4) will be (2,2)