You can put this solution on YOUR website! I'm assuming you meant log base 4:
log4(x+16) + log4(x+4) = 3
log4(x+16)(x+4) = 3
(x+16)(x+4) = 4^3
x^2+20x+64 = 64
x^2+20x = 0
x(x+20) = 0
x = {-20, 0}
The -20 won't work (extraneous) throw it out leaving:
x = 0
