Answer:
The answer to your question is 4699 g
Step-by-step explanation:
Data
mass of Li₂CO₃ = ? 
concentration = 12.7 M
volume = 5 l
Process
To solve this problem use the formula of Molarity and solve it for moles.
1.- Write the formula of Molarity
Molarity = moles / volume
-Solve it for moles
moles = Molarity x volume
2.- Substitute the values
moles = 12.7 x 5
3.- Result 
moles = 63.5 
4.- Calculate the molar mass of Li₂CO₃
Li₂CO₃ = (7 x 2) + (12 x 1) + (16 x 3)
 = 14 + 12 + 48
 = 74 g
5.- Use proportions to calculate the moles
 74 g of Li₂CO₃ -------------- 1 mol
 x -------------- 63.5 moles
 x = (63.5 x 74 ) / 
 x = 4699 g