What is the equation for determining the average atomic of an element

If you're asking for average atomic mass. You the multiple the two isotopes mass numbers by the relative abundance they give you then you add the products. So if we have isotopes carbon - 16 and carbon - 17. relative abundance for both is 50.

it's (16.000 x 0.50) + (17.000 x 0.50). then you add the products and get your answer