could someone explain this to me please

I think this explanation should be better

See the bracket where the identity is given in the terms of a and b?

Observe carefully if you see then a=(7-x)^(1/2) and b= (x-5)^(1/2) now the denominator here is = a+b

When you split the numerator as provided by the identity the bracket of (a+b) will cancel and the denominator will disappear.

Try breaking down the numerator yourself according to the given identity if you still do not understand. You will see it.

Now for the second last line. How did we get it you ask?

7-x+x-5 = 2 and then this 2 on the LHS will cancel with the 2 on the RHS

Hence we get the second last line.