The molarity of a solution obtained by mixing 750 mL of 0.5 (M) HCl with 250 mL of 2 (M) HCl will be :

Total moles = 0.875 moles
Total volume = 750 + 250 = 1000ml = 1L

$\text{ Molarity }=\frac{\text{ Moles }}{\text{ Volume }}=\frac{0.875}{1}=0.875\mathrm{M}$