Okay so Tyler is 9 right? and his sister is 6 years less than TWICE of his age. so twice of his age is 18. (9 x 2 = 18) and she is 6 years less (18-6 =??) thus, Olivia is 12. a way you can write an expression is:
(9 x 2) - 6 = 12.
fyi: parentheses means you do whatever equation that in in the parentheses first.
hope I helped! Good Luck!