I never said this. What are you talking about?
The gradient of any line is mx + c = y, that’s grade school math. That’s all you used to work it out and you’re claiming you used calculus.
Alas, the answer for the coveted star was:
Multiply the equation with d/dt: (d/dt) * t*sqrt(26) = dt * sqrt(26) / dt.
dts cancel out, therefore the derivative is sqrt(26).
Still a very unnecessary (and simple) application of calculus, but at least it would’ve shown that’s whatcha did.