In order to do this you have to find numbers that can be multiply and get those numbers.... for example in 15 if you multiply 5 times 3 you get 15 so lets use that (5x )(3x ) now lets look at the last digit.. we can use 6 times 8 and it gives you 48.. for the signs we can see the first is negative and the second positive his means that both of the sings in here will be negative, so lets put all of that in now (5x-6)(3x-8) lets see if it works to get the middle number now. 5x times 3x is indeed 15x^2, 5x times -8 is -40x and -6 times 3x is -18x meaning that if we add those (snce they are both negative) we will get -58x.. now for our last digit -6 times -8 is indeed 48,... so your answer is (5x-6)(3x-8)
Hope this helps