If the call cost 35 cents to use the card that leaves $12 that was from minutes. There is 4 quarters in a dollar, 12x4 is 48.
VERIFY:
$1=4
$2=8
$3=12
$4=16
$5=20
$6=24
$7=28
$8=32
$9=36
$10=40
$11=44
$12=48
It's hard to type and hard to read the "inverse tangent" function, as you've seen (above).
So, use "arctan x" instead.
Then the problem becomes: "differentiate cos (arctan x)."
You must apply first the rule for differentiating the cosine function, and next apply the rule for differentiating the arctan function:
(d/dx) cos (arctan x) = - sin (arctan x) * [1/(1+x^2)]