Question 353161
First name your variables.
Let S be the amount of soybean meal.
Let C be the amount of corn meal. 
Now find the total weight equation.
1.{{{S+C=240}}}
Now find the protein concentration equation.
{{{12S+6C=7(240)}}}
2.{{{6S+3C=840}}}
Now you have 2 equations with 2 unknowns. 
Multiply eq. 1 by (-3) and add the equations together.
{{{-3S-3C+6S+3C=-720+840}}}
{{{3S=120}}}
{{{highlight(S=40)}}}lb.
Then from eq. 1,
{{{40+C=240}}}
{{{highlight(C=200)}}}lb.