To know if an equation is balanced you need to check and see how much of each molecule is on either side of the arrow. Right now you have 1-Ca, 2-H, 2-Cl on the left side of the arrow and 1-Ca, 2-Cl, and 2-H on the right side too. Because all the molecules are equal on both sides this means that the equation is balanced. So in front of the CaCl2 there is an assumed coefficient of 1. The answer is 1.
