There is a quick way to solve this using the (–b+-sqrt(b^2-4ac))/2a but to solve it by trial and error is better.
We have the coefficients of x^2 which when multiplied together gives 16 and the constants when multiplied together to gives 9. But the sum of the cross products i.e. the first coefficient of x^2 with the second constant plus the second coefficient of x^2 with the first constant equals -40.
What first comes to mind is 4 times 9 and 4 times 1, so for the sum to be -40 we have minus 9 and minus 1.
