You add 3.5 L of an HCl solution of unknown concentration to 2.0 L of 0.5 M HCl and 4.5 L of water. The final concentration of HCl was 1.5 M. What was the unknown concentration of the initial HCl solution?

(3.5L*xM) + (2.0L*0.5M) = (10L*1.5M)

Solve for x molar.