Find the length x in the triangle. express your answer in simplified radical form. side 1=10. side 2=8. side 3=x

There's not enough information given to find a unique answer. 
With sides of 10 and 8, an infinite number of different triangles
can be built, each with a different third side.  The third side
can be any length between 2 and 18 .

There may be a picture that goes along with this question, with some
more information in the picture than what has been given here. Like
for example, it may be some special kind of triangle.

3x+9+4x+x=8x+9that's the answer for your question.