Which is a factor of 2n^2-5n-42a 2n-7 b 3 - 2nc 3-2n^2d 4-2n^2

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.