Question

The average human requires 120.0 grams of glucose (C6H12O6) per day. How many grams of CO2 ( in the photosynthesis reaction) are required for this amount of glucose? The photosynthetic reaction is: 6 CO2 + 6 H2O = C6H12O6 + 6 O2

Answer 1

Your problem is a stoichiometry problem. Here is a worked example. Just follow the steps. Post your work if you get stuck.
http://www.jiskha.com/science/chemistry/stoichiometry.html

Answer 2

I'm having trouble putting the work into diminutional analysis

Answer 3

You look at the factor and make it so that the denominator is in the unit you start with and the numerator is the unit you want to keep. The numbers are the coefficients in the balanced equation.
For example, in the equation
H2 + Cl2 ==> 2HCl, if we start with 10 moles H2 and want to convert to moles HCl, we have
10 moles H2 x (2 moles HCl/1 mole H2) = 20 moles HCl. You notice that moles H2 cancel (that's what you started with) and it leaves moles HCl (what you want for the answer).