Cary and her friends tasha both had 20 tickets each to play games. cary used x of hers and tasha used 2 times as many as cary. when cary's dad doubled the combined amount they had left, they ended up with 39 tickets. how many tickets(x) did cary use?

Cara   had (20-x) tickets that left.
Tasha had (20-2x) tickets that left.

Tasha and Cara together
(20-x) +(20-2x)=20+20-x-2x=40-3x 

Tasha and Cara together had  (40-3x)  tickets that left.

Dad doubled it    
2*(40-3x)

Now they have
(40-3x) +2*(40-3x)=39

40-3x+80-6x=39
120-9x=39
9x=120-39
9x=81
x=9

Cary used 9 tickets.

The correct answer is -4