Trigonometry equation $\sin(x)+\cos(x)-\tan(x)=0.4$ There's some way to find $x$ here ?

$$\sin(x)+\cos(x)-\tan(x)=0.4$$
Avoid squaring wherever practicable as it immediately introduces extraneous roots 

Using  Weierstrass substitution we have

$$\frac{2t}{1+t^2}+\frac{1-t^2}{1+t^2}-\frac{2t}{1-t^2}=0.4$$ where $t=\tan\frac x2$

On rearrangement we have a Biquadratic Equation in $t$ with no extraneous root, find the answer here
Convert $\tan x = \frac{\sin x}{\cos x}$

Now we can write $\cos x$ as $\sqrt{1-\sin^2x}$ 

Now you will get an equation in sin x which you can solve.