Split the middle term.
(2n²-5n)-42
Multiply the coefficient of the first term (2 is the coefficient of 2n²) by the constant (-42, it's constant because there are no variables attached).
2 x (-42) = -84
Find the factor of -84 that equals your middle term (-5)
The factors of 84 are as follows (remember that if either number is negative, you'll get -84):
84*1
42*2
28*3
21*4
14*6
12*7
What two numbers, when added or subtracted, equal the middle term? 12-7 = 5, but your number is a negative, so how do you do get that 5 to be negative? -12+7 = -5, but make that easier to understand 7-12 = -5, and now add your variable back to get 7n-12n = -5n
Rewrite your equation using the split middle term.
2n²+7n-12n-42
Pull out the factors and add up the first two terms.
n*(2n+7) = 2²+7n
-6(2n+7) = -12n-42
The numbers inside the parenthesis give you one half of your new equation, combine the numbers outside the parentheses to get your second half.
(n-6) (2n+7)
In regards to your answer, there are three possibilities:
1) You wrote the initial equation incorrectly
2) You wrote the answers incorrectly
3) Whoever created the question wrote the equation or the answers wrong, and the answer is not present.