corrections

def collinear(x1, y1, x2, y2, x3, y3): # <-- third point
    m = 0

    a = x2 - x1
    b = y2 - y1

    if a != 0:
       m = b/a # <-- instead of a/b
       print(m) # <-- instead of return m

    if (y3 - y2)*(x2 - x1) == (y2 - y1)*(x3 - x2):
       print("True")
    else:
       print("False")

    #return m