I'm having trouble understanding how to check the second order conditions for my unconstrained maximization problem.

This is the entire problem: Alicia wants to maximize her grade, which is a function of the time spent studying ($T$) and the number of cups of coffee ($C$) she drinks. Her grade out of 100 is given by the following function.

$G(T,C)=50+10T+16C-({T}^{2}+2TC+2{C}^{2})$

In the first order conditions, I find the partial derivatives and set them equal to zero. I get the following two equations:

$10-2T-2C=0$ and $16-2T-4c=0$. The first equation was the partial derivative with respect to $T$ and the second equation was the partial derivative with respect to $C$. Solving these two equations, I find that $C=3$ and $T=2$.

Now, I need to check the second order conditions. I know that the second partial derivative with respect to both $T$ and $C$ should be negative. This checks out. I get -2 from the first equation (with respect to $T$) and I get -4 from the second equation (with respect to $C$). The last thing I need to do with the second order condition is multiply these two together (which yields 8) and then subtract the following:

${\left(\frac{{\delta}^{2}G}{\delta T\delta C}\right)}^{2}$

Please forgive me if this formula isn't displaying correctly. I tried using the laTex equation editor, but I'm not sure if it worked. Anyway, I need to know how to derive this. What is it asking for? I know that this part should be -2 squared, which is 4. Then, 8-4=4, which is positive and tells me that the second order conditions are met.

But where is the -2 coming from? I know within both of the equations, there are a few -2's. But, I'm not sure exactly where this -2 comes from.