so one should be plus and one minus to get the -4. now, you want to come up with two numbers that when multiplied gives you -4.
what are our options, we can have 1 times 4 or 2 times 2, with the proper signs attached. if you can't see which to use, you can go by trial and error.
lets do 1 and 4
(2x 1)(x 4)
can we get what we want from this?
well, the middle term is given by the product of the inner terms plus the product of the outer terms (remember FOIL)
multiply the outer terms, we get 8x
multiply the inner terms, we get 1x
can we make -7x with this? you bet we can, we got lucky. -8x + x = -7x. so this tells us how to put the signs in
(2x + 1)(x - 4) .......put the negative on the 4 so as to get -8x and the plus sign on the 1 to get the positive x.
so we have (2x + 1)(x - 4) = 0
can you continue? (remember, if we have two numbers being multiplied result in zero, then one number or the other must be zero)