we cannot factor out a 4 since 15 is not divisible by 4. we must factor by grouping. look for 2 numbers whose product equals the product of 4 and -15 their sum needs to be equal to positive 4 the first clue is the product of the numbers is negative, so a and b must have different signs but a + b is equal to 4 so, the difference of their absolute values must equal 4 