Answer
$\frac{d}{dx}(\tan x -x+C)$
$=\sec^2 x-1$
$=\tan^2 x$ (as needed).
Work Step by Step
Differentiating the right side:
$\frac{d}{dx}(\tan x -x+C)$
$=\sec^2 x-1$
Recall the identity $\tan^2 x + 1 = \sec^2x$
$=\tan^2 x$ (as needed).