You likely need to load `ImplicitEquations`

, which defines `\Equal[tab]`

.

As for what to make greater, presumably your 0.1, but it is your exploration, so I’ll defer. But this choice creates a graphic:

```
using ModelingToolkit
using Plots
using ImplicitEquations
@variables x y;
f(x,y)= sqrt((x^2+144)+(y^2+144))
let ex = Differential(x)(f(x, y))
func_ex = build_function(expand_derivatives(ex), x, y)
@eval ∂xf(x, y) = ($func_ex)(x,y)
end
using Plots
using ImplicitEquations
plot(∂xf ⩵ 0.1, xlims=(-10, 100), ylims=(-100, 100))
plot!(f ⩵ 20, xlims=(-10, 100), ylims=(-100, 100))
```