6x^2+bx+6=(3x+m)(nx+3) for all values of x what is the value of b?

First, Delete these brackets on the right side:

Left side and right side are equal, so each parameters have to be equal. Next to "x^2" you've got parameters "6" and "3n".  So:
3n=6   /:3
n=3

Free terms are "6" and "3m", so:
3m=6  /:3
m=2

Next to "x" you've got  "b" and "9+nm" .  We know, that m=2 and n=3  so:
b= 9+nm
b=9+2*3=9+6
So b=15