Square roots

[Guest]

Given (3+ square root sign 7)(3-square root sign 7)
The correct solution is___

Choices are 4, 5 or 2

I have NO idea how to even start this one.

Can someone help me?

 

[Denis]

Given (3+ square root sign 7)(3-square root sign 7)
Choices are 4, 5 or 2

Show these like this: (3 + sqrt(7))(3 - sqrt(7))

Rule: sqrt(n) * sqrt(n) = n
Example:
sqrt(4) * sqrt(4) = 4
sqrt(4)=2 and 2 * 2 = 4, right?

So sqrt(7) * sqrt(7) = 7;
can you finish it now?