Initially it helps to read over the question a couple of times underlining/ highlighting all important information ie. volumes and concentrations. Next it always helps to have a chemical equation to work with so if you're not already given one in the question write one out, ensuring it is balanced. The next step is to work out the number of moles of HCl- (140.1)/1000 = 1.410-3 mol - and use this to work out the number of moles of CaCO3 (using mole ratios - in this case 1:2) - 1.410-3/2 = 710-4 mol. From this we can work out the concentration of CaCO3: [CaCO3] = (7*10-4 * 1000)/ 25 = 0.028moldm-3.