Let's use the quadratic formula to solve for x:
Starting with the general quadratic

the general solution using the quadratic equation is:

So lets solve ( notice , , and )
Plug in a=16, b=48, and c=40
Square 48 to get 2304
Multiply to get 
Combine like terms in the radicand (everything under the square root)
Simplify the square root (note: If you need help with simplifying the square root, check out this solver)
Multiply 2 and 16 to get 32
After simplifying, the quadratic has roots of
or 
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