Question 69347
You have the first problem down. If you know the population in 1990, in both algebraic and numerical form, then you could find the population in 1980. The population in 1990 equals (according to part a) P+(P*.12) then we can find P (the population in 1980. 
Population in 1990:{{{ P+P*.12=632910}}} add P and .12P
{{{(cross(1.12)P)/cross(1.12)=632910/1.12}}}Divide both sides by 1.12
{{{P=565098.21}}}Note: the decimal part is a result of division and not actual result (cant have a fifth of a person), so you might want to round.
By the way you have the right idea for part b, you just subtracted .12P instead of adding it. Adding it to P allows you to isolate P.