1)

One way is go ahead and multiply those in the numeratior,

= [6sqrt(6*24)] / [-sqrt(18)]

Then see if you can factor the new numerator so that you can cancel the denominator,

= [6sqrt(6*3*8)] / [-sqrt(18)]

= [6sqrt(8)] / [-1]

= -6sqrt(4*2)

= -6*[2sqrt(2)]

= -12sqrt(2) -------answer.

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2)

In cases like this, rationalize the denominator, or multiple the numerator and the denominator by the conjugate of the denominator.

The conjugate of the denominator here is [1 -sqrt(a)], so,

= [sqrt(a) *(1 -sqrt(a))] / [(1 +sqrt(a))*(1 -sqrt(a))]

= [sqrt(a) -a] / [1 -a] -------------answer.