Question 903100
To solve, we use the distributive postulate. A postulate is a statement that is accepted as true.  A distributive postulate would be written like this :

a(b+c) = ab + ac 

sqrt 7 ( 6sqrt3 - 6sqrt7)

= 6 sqrt 21 - 6 sqrt 49
= 6 sqrt 21 - 6*7
= 6 sqrt 21 - 42

Explanation : when we multiply radicals, we just multiply the radicands. A radicand is the number inside the radical sign. Therefore, sqrt of 7 multiplied by sqrt of 3 is sqrt of 21. We cannot simplify sqrt of 21 any further as the factors for 21 are simply 3 and 7. These are not square numbers. 
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