Answer :

( x-4) (3x -y)

multiply the first X with the other two terms in the other paranthesis

so x * 3x and x * -y

and ull get 

3x^2 - xy 

now multiply the -4 with the two terms of other paranthesis

so -4 * 3x and -4 * -y 

and u get -12x and 4y

cant simpligly so 

3x^2 - xy -12x + 4y