I think you do $15 x 16 cents = 900minutes / 16 cents = 60 minutes.
And then do $10.52 x 16cents = 168.32
Subtract 900-168.32 = 731.68minutes / 16cents= 45.73 minutes not used
60minutes - 45.73 =14.27min used.
Sorry if this isnt correct.
I would have used graph b. Because it shows the decrease more than graph a.
Use double angle identity

We can relate '6x' to '3x' in the same way since 6 is 2*3.

Now sub into left side of identity. Simplify until it equals right side.