Simplify:
​$$a^2+9b^2+c^2-6ab+6bc-2ac=$$​​

Correct option is C

Given:

Expression : $$a^2 + 9b^2 + c^2 - 6ab + 6bc - 2ac$$​

Formula used:  

(a + b + c)² = a² + b² + c² + 2(ab + bc + ca)

Solution: 

$$a^2 + 9b^2 + c^2 - 6ab + 6bc - 2ac \\ \ \\ =a^2 + (-3b)^2 + (-c)^2 +2(a)(-3b) + 2(-b)(-c) + 2(a)(-c)\\ \ \\ \text{ Comparing this with the formula of }(a+b+c)^3 ;\\ \ \\ = (a - 3b - c)^2$$​