Question 529966
OK
x^2+6x=16
Let's add 9 to each side (half of the x coefficient squared)

x^2+6x+9=16+9
(x+3)^2=25  take sqrt of each side
x+3=+-5
x=-3+-5
x=-8 and
x=2

This may be a useful tutorial for you

Take the general form of the quadratic:
Ax^2+Bx+C=0
(1) Subtract C from each side
Ax^2+Bx=-C
(2) Divide each term by A
x^2+(B/A)x=-C/A
(3) Take half of the x coefficient, square it and add it to each side
Half of B/A=B/2A and when we square it, we have B^2/4A^2. Now we add it to each side:
x^2+(B/A)x+B^2/4A^2=(-C/A)+B^2/4A^2

Now the left side is a perfect square:
(x+B/2A)^2=(B^2-4AC)/4A2  
(4) Take the sqrt of both sides:
(x+B/2A)=+-SQRT(B^2-4AC)/2A or
x=(-B+-SQRT(B^2-4AC))/2A

Hope this helps---ptaylor