Question

Square root of 45 -3 square root of 20

Answer 1

square root of 20 is prime factor of 20 i.e 2×2×5
now √2×2×5 = 2√5 because pairs came out from the 
under root 

Answer 2

Given
$\sqrt{45} - 3 \sqrt{20}$
$\sqrt{9*5} - 3 \sqrt{4*5}$
$3\sqrt{5} - 3*2 \sqrt{5}$
$3\sqrt{5} - 6 \sqrt{5}$
$-3\sqrt{5}$