In each part, we are taking square root, but the number under the square root is different. We need to simplify the radicals first and see if we can combine them. We recognize that there is a √7 term involved in each item.

`6sqrt(7) - sqrt(28) + 3sqrt(63) `

`= 6sqrt(7) - sqrt(4 times 7) + 3sqrt(9 times 7)`

`= 6sqrt(7) - 2sqrt(7) + 3(3 sqrt(7))`

`= 6sqrt(7) - 2sqrt(7) + 9sqrt(7)`

`=13sqrt(7)`

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