3Mg(OH)2 + 2H3PO4 Which lists the numbers of each atom the other half of the equation would contain?

In the millikan oil drop experiment they determined that every drop had a charge which was a while number multiple of -1.60x10^-19. if a drop has a total charge of -9.60x10^-19 then how many excess electrons are contained within the drop?

Ineed to find the answer of this question because i dont understand it

Draw the lewis dot structure for each of the following polyatomic ions