I need help on this.

26.88 mm

Step-by-step explanation:

4.2 x 12.8 = 53.76/2 = 26.88

28.3 i think

Step-by-step explanation:

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)