Get the average of the two x and two y separately.
x = (8 + 10)/2
the 8 and 10 are the x values from the given endpoints, so...
x = 18/2
x = 9
next one is for the y... (get the y values of the given endpoints just like in the first step)
y = (13 + 9)/2
y = 22/2
y = 11
Final Answer...
Midpoint ( 9, 11 )