how do i find the missing angle if i know that the tanx=34/16

If you are talking about a right-angled triangle where x is one of the acute angles, then the other is given by tan^-1(16/34)

$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{16}}}{{\mathtt{34}}}}\right)} = {\mathtt{25.201\: \!123\: \!645\: \!475^{\circ}}}$$

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Alternatively, you can do it by taking 90°-tan^-1(34/16)

$${\mathtt{90}}{\mathtt{\,-\,}}\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{34}}}{{\mathtt{16}}}}\right)} = {\mathtt{25.201\: \!123\: \!645\: \!475}}$$

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I've interpreted the words "missing angle" as the other acute angle in a right-angled triangle.  Melody is possibly more correct in interpreting it as just the value of x. (Though you should have 16 not 15 Melody!).

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$$\underset{\,\,\,\,^{\textcolor[rgb]{0.66,0.66,0.66}{360^\circ}}}{{tan}}^{\!\!\mathtt{-1}}{\left({\frac{{\mathtt{34}}}{{\mathtt{15}}}}\right)} = {\mathtt{66.194\: \!056\: \!481\: \!542^{\circ}}}$$

 

when looking for the angle you must use inverse tan (or arc tan).  They are the same thing.

If using the web2 calc it is atan

so to get my answer, I just inputted        atan(34/15)=