Verified answer

a² - b² = (a - b)(a + b)

1)

62/√93 = 62*√93/(√93*√93) = 62√93/93 = 2√93/3

2)

4/(4 + a) = 4(4 - √a)/((4 + √a)(4 - √a)) = 4(4 - √a)/(4² - √a²) = 4(4 - √a)/(16 - a)

3)

2/(√5 - √7) = 2(√5 + √7)/((√5 - √7)(√5 + √7)) = 2(√5 + √7)/(√5² - √7²) = 2(√5 + √7)/(5 - 7) = -(√5 + √7) = -√5 - √7