The distribution of dog weight is normally distributed with a mean of 18 pounds and a standard deviation of 3 pounds. therefore, 84% of dogs weigh below how many pounds?

Here you have to work with z-score table.

Formula: z=(x-μ)/σ, where x is the # of dogs, μ = mean and σ = standard deviation.

You look up in the score table for which value, z corresponds to 0.84 and you will find that z = 0.7995 for a probability of 84% 

0.7995 = (x-18)/3 → x =20.39 or 84% weighs under 20.39 pounds

Sorry i didn’t know i had

3x+9+4x+x=8x+9that's the answer for your question.