a) (5√2-√18)√2 = 5√2 √2 - √18 √2
= 5√4 - √36
= 5 * 2 - 6
= 10 - 6
= 4

б) (√7+√6)² = (√7+√6)(√7+√6)
= (√7 √7) + (√7 √6) + (√6 √7) + (√6 √6)
= 7 + 2√42 + 6
= 13 + 2√42