ıf tan(x)=4/5 .. x = ?

Use tan^-1, which is atan on the calculator.

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{4}}}{{\mathtt{5}}}}\right)} = {\mathtt{38.659\: \!808\: \!254\: \!09^{\circ}}}$$

This is the first quadrant solution.  There is also a third quadrant solution obtained by adding 180° to this.

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}{\left({\mathtt{38.659\: \!808\: \!254\: \!09}}^\circ\right)} = {\frac{{\mathtt{4}}}{{\mathtt{5}}}} = {\mathtt{0.8}}$$

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}{\left({\mathtt{180}}{\mathtt{\,\small\textbf+\,}}{\mathtt{38.659\: \!808\: \!254\: \!09}}\right)} = {\frac{{\mathtt{4}}}{{\mathtt{5}}}} = {\mathtt{0.8}}$$

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Ummmm.  what?

ı need to find the angle

Oh.......................

how can ı find it ??