Question

Limit as x approaches b is (b-x)/(square root of x - squareroot of b).

Answer 1

Have you considered rationalizing the denominator? Or, perhaps factoring the numerator?

\(b-x\;as\;a\;difference\;of\;squares,\;gives (\sqrt{b}+\sqrt{x})(\sqrt{b}-\sqrt{x})\)

Answer 2

i multiplied by the conjugate of \[\sqrt{x}-\sqrt{b}\]

Answer 3

Either way. :-)