Tolong dengan caranya

18.
√72 + 2√8 – 7√2
= √(36 x 2) + 2√(4 x 2) – 7√2
= 6√2 + 2(2√2) – 7√2
= 6√2 + 4√2 – 7√2
= (6 + 4 – 7)√2
= 3√2

19.
√7(√7 – 2)
= (√7)(√7) – 2(√7)
= 7 – 2√7

20.
(√3 + 2)(√3 – 2)
= (√3)² – 2²
= 3 – 4
= -1