What is the ratio between H2O and M2CO3?
The question is pretty vague but I will answer it based on what you have given. First of all M is not an element so I'm not sure what M2CO3 is but I will assume that it is perhaps a variable for any element that can form a carbonate (CO3) salt. There is no chemical reaction listed but since the leading coefficients for both chemical compounds is one then you must assume that the H2O and M2CO3 are in a 1:1 molar ratio. This means that for every 1 mole of water there is 1 corresponding mole of M2CO3.