Check your question! Could be 5x+3<3x-9 or 5x+3<3x+9.
5x+3<3x-9, 2x+3<-9, 2x<-12, x<-6;
5x+3<3x+9, 2x+3<9, 2x<6, x<3.
In inequalities, solutions are not usually unique values but rather a range of values.
In this case x<-6 means any value of x between -∞ and -6 (x must be less than -6 and not equal to -6).
This is the interval (-∞,-6).
x<3 means any value of x between -∞ and 3 (excluding x=3).
This is the interval (-∞,3).