"Molarity"=("moles of solute")/("liters of solution")
A "3.5 M CaCl"_2" solution contains "3.5 moles" "CaCl"_2"/L solution".
We need to convert moles "CaCl"_2 to grams. We do this by multiplying moles "CaCl"_2 by its molar mass ("110.978 g/mol").
3.5color(red)cancel(color(black)("mol CaCl"_2))xx(110.978"g CaCl"_2)/(1color(red)cancel(color(black)("mol CaCl"_2)))="300 g CaCl"_2" rounded to two significant figures
Since "300 g CaCl"_2" are needed to make "1 L" of a "3.5 M" solution, "600 g CaCl"_2 are needed to make "2 L" of a "3.5 M" solution.