The answer is 1.3 x 10^-29.
Essentially you are using Hess's Law with K values. In order to get the final equation, you must multiply the coefficients of the first equation by 2. Therefore, what you do to the K value is you raise the constant to 2. So you have, 5.3^2.
Next, you know you need to flip the second equation in order to obtain the final equation, but you aren't multiplying the coefficients, you don't need to. So you raise the K value to the -1 power. So you do, (2.1 x 10^30)^-1.
Multiply the two K values together to get your final equilibrium constant for the final equation.
(5.3^2) x (2.1 x 10^30)^-1
Hope that helped!
