Since one atom of Rb turns into one atom of Sr, there were initially 87+ 947= 1034 atoms of Rb. Now there are 947 atoms left.

Saying that it has a half life of 48.8 billion years means the amount is multiplied by 1/2 every 48.8 billion years. If T is the number of billions of years that have passed, then T/48.8 is the number of "half lives" and so that amount left after T billions of years is where "A" is the initial amount. Your equation is

Solve that for T.