What is the square root of 45 ​

6.7082039324993691

$\sqrt{45} = 6.7082039324993691$

It also can be expressed as: 

√45   =  √9 * √5  =   3 √5

Well if you want the EXACT answer it's 3 times the square root of 5. 
- How you get that:
1. Separate the 45 within the square root into two terms (9 times 5).

2. 9 is a perfect square so you can extract it out as three, leaving the five inside. Thus you get the answer above.