Hi, so im trying to find the equation of one median to find the centroid, but im stuck at trying to find slope because slope=6/0

here's my work so far;

MidpointXZ = B (0+8/2, 0+(-4)/2) = B(4,-2)

SlopeYB = 4-(-2)/4-4=6/0

this is where im stuck. is slope now undefined? what's the next step

thanks for your help!