the answer
the main rule for a square root calculus is given by √A*√B = √A*B, for all value positive of A and B
so we can extended the given value of the fonction as
√x-7 *√x+1 = √x-7 *√x+1 = √(x-7 )* (x+1)
but the extended form of (x-7 )* (x+1) is (x-7 )* (x+1) = x² + x -7x -7= x²-6x -7
therefore, √(x-7 )* (x+1) = √(x²-6x -7), the value which is equivalent to the radical expression is B)√x^2-6x-7, when x≥7