Your first equation is correct, but the second is not. Let's look at the statement:
The energy drink has 6 more than twice the number of mg of caffeine as compared to the cola.
The energy drink:
than twice the number of mg of caffeine as compared to the cola:
Putting this together into an equation, we have:
Now, substitute this into the first equation for to get:
Solve this for , then use the equation to find .