Answer:
(1) You can try to calculate length of AB, BC, CD, AD, AC, BD to make sure the order of vertices. (other way: you can plot them to see form of trapezoid and then find out positions of 4 vertices)
(2) From (1), identify 2 bases of trapezoid: AB and CD
=> Length of mid segment = (lengAB + lengCD)/2
=[ sqrt[(0-3)^2+(5-3)^2] + sqrt[(5-(-1))^2+(-2-2)^2] ]/2 = ~5.4
Other way
Define two points create mid segment P1 and P2
P1 is middle of AD = (A+D)/2 = (-0.5,3.5)
P2 is middle of BC = (B+C)/2 = (4,0.5)
=> Length of P1P2 = sqrt[(-0.5-4)^2+(3.5-0.5)^2] = 5.4
