Solving Integral Equation: x(t) = -8?

[brusier]

Homework Statement

I integrated dx/dt = x^2+(1/81)

Homework Equations

My result; 9(arctan(9x))= t+C needed to be solved for initial condition x(0)=-8
and fit into x(t) = format

The Attempt at a Solution

I cannot figure out why the computer is not accepting my solution:
arctan (9x)= (t-14.012)/9

 

[PhysicsMark]

Wouldn't that be:

$$x(t)=\frac{(tan(\frac{t-14.012}{9}))}{9}$$