The causal impact of tar on cancer is:
P(cancer|do(tar)) - P(cancer|do(no tar))

where,

P(cancer|do(tar)) = P(cancer|tar,smoking) * P(smoking) + 
                    P(cancer|tar,no smoking) * P(no smoking)

P(cancer|do(no tar)) = P(cancer|no tar,smoking) * P(smoking) + 
                       P(cancer|no tar,no smoking) * P(no smoking)