Hello

I don't now if this will help, but I do it this way. It doesn't take ages.

You know you have either:

1,15 or 3,5

and your looking for a difference of 11

Write out:

(12x )(x )

(6x )(2x )

(4x )(3x )

It doesn't tke long to work out the difference of the possible products and see that 4x5 - 3x3 = 11

So you have

(4x - 3)(3x + 5)