How to solve algebra equation?

Question

Solve: 
If x^2-13x+42=(x+a)(x+b) then 3a-b=?

Answer 1

The expression is  

Now finding the factors for 

Now  

So, the value of  a = - 7  and b = - 6 or  a= -6 and b  = - 7.

Case 1:

The value of  3a-b ,for a= -7 and b= -6:

3(-7) - (-6) = -21 + 6 = -15.

So, the value of 3a - b is -15 when a=-7 and b= -6.

Case 2:

The value of  3a - b ,for a= -6 and b= -7:

3(-6) - (-7) = -18 +7 = -11.

So the value of 3a - b is -15 when a=-6 and b= -7.

Hence, the value of 3a - b is -15 or -11.