Question

If 2.5 g of element A combine with 5 g of element B to form compound AB how many grams of B are needed to form compound AB2? Which law did you use to answer this question ? My answer is 7.5 g. Im not sure if this is right. Please help this is due Thursday morning C an you please show me how to do it so I can do it next time teacher ask. I'm so confuse don't wanna give up. PLS ANSWER ILL WAIT. THANK YOU SO MUCH