Defiing function using if-statement

def roots4(a,b,c,d):
d = b * b - 4 * a * c
if a != 0 and d == 0:
        roots4(a,b,c,d) 
        x = -b/ (2*a)
if a != 0 and d > 0:
        roots4(a,b,c,d)
        x1 = (-b + math.sqrt(d)) / 2.0 / a
        x2 = (-b - math.sqrt(d)) / 2.0 / a
if a != 0 and d < 0:
        roots4(a,b,c,d)
        xre = (-b) / (2*a)   
        xim = (math.sqrt(d))/ (2*a)
        print x1  = xre + ixim
        strx1 = "x2 = %6.2f + i %6.2f" %(xre, xim)
        print strx1 

This is part of my code for a project. What im trying to do is define "roots4(a,b,c,d)". For example, in the case that "a != 0 and d == 0" then roots4(a,b,c,d) shall then go to to finding x by solving equation "x = -b/ (2*a)". And so on... i dont know what im doing wrong. Any tips?