I am assuming "1xa" is expressing "1 multiplied by a", using an 'x' for multiplication.  The symbol '*' is an algebraic notation allowed for multiplication but any reference to 'x' is specifically meaning 'the variable "x"'. 

"3^2-1xa" is best expressed as "3^(2-1)a" and the "2-1" points out that the '3' in the denominator has been cancelled out.