To find the midpoint of a line use the midpoint formula which is M = (x1+x2/2 , y1 +y2/2) After plugging in the coordinates for your variables you should get (-1+5/2, -1+2/2) This simplifies to (2, 1/2) which is the coordinates for the middle point of the base XY.
Think about it do you think it's subtraction or addition?
So your formula is y1 - y2 divided by x1 - x2. Start off with the -5, 14 as your x1 and y1 because its the biggest out of the other one. So do 14 - 2 = 12 so thats your top part and -5 - -1 which is -4 so your fraction will be 12/-4 or you have -3 as a whole number
It is stil half becuase the real probability doesnt change