Solution:

We have given that point let A(1,2) and point B(3,4) and

the equation is 3x-4y+a = 0

putting both values of A and B in LHS of equation, we get

3*1-4*2+a = 0 --------(1)

a = 5

3*3-4*4+a = 0 ------------(2)

a = 7

From both equation we get the values of a

**a =5 and a= 7**

**So the correct answer is (b)**