Question 815056
Tamic?  At least you did not say "tannic".  
I would only question what kind of concentration you mean, whether w/w or w/v or v/v.  7% solution is significant enough strength that density maybe be very important for the calculation.  If the percents are v/v, then you simply can use


{{{4*7/(4+x)=5}}}  where x is liters of water to add.