The equation you wrote looks sensible, but you need more than 1 equation to solve the problem.
If you define:
F = the amount of flour you added
W = the amount of water you added
M2 = the amount of mixture 2 added
The total amount of water in your output is
And the total amount of Flour is
To end up with mixture1, you need to have the same amount of flour and water
You are also told to use M2 for 1/6 of your input, which means there should be 5 units of standalone (flour + water) for each 1 units of M2.
Now all you need to do is decide how much mixture to make, to do this set an arbitrary amount of one of the ingredients eg
Then solve the other two equations simultaneously.
Which has a solution of
F = 8/3
M2 = 1
NB post was edited to fix a mistake earlier