The best way to tackle this is to put the answer that you know into a binomial factor. So the solution is x = 4. The binomial factor is (x - 4)
Now you can set up the other binomial factor. You have been given a huge hint. The third term is 28 and one of it's 2 factors is 7. So this is what you know so far
(x - 4)( x 7) But what is its sign? The 28 is positive. So the 7 must be minus giving you
(x - 4)(x - 7) Are we done yet? No There's a 3 on the x^2. Does the 3 go with the x on (x - 4) or does the 3 go with the (x - 7)? We have been told that (x - 4) is one of the factors. No mention of a three. So we should try x - 7 first and write it like this.
(x - 4)(3x - 7)
I think if you remove the brackets, you will find that the middle term is - 19 (-12 - 7 = -19 ) which is exactly what it should be.
Answer (x - 4)(3x - 7)