We must first consider the relative valencies of the elements. Recall that the formulae of the compound must involve an exchange in the valencies of the two combining species. Hence the first step in solving the problem must be to put down the individual valencies of the both elements then exchange thief valencies to obtain the actual formula of the compound formed by a combination of the both species as we shall show below;
In this case, when forming oxides from a metal and oxygen, for us to find out each element's subscript, we must exchange them as shown below, considering +5 for scandium:
