Question 118078
I suspect there was a diagram that accompanied this problem, because you left out some rather important details.  First, is this a right triangle?  And, if it is a right triangle, is side 1 one of the legs or is it the hypotenuse?

Assumption 1:  It is a right triangle and x is one of the legs:


{{{x=sqrt(12^2-8^2)=sqrt(144-64)=sqrt(80)=4*sqrt(5)}}}


Assumption 2:  It is a right triangle and x is the hypotenuse:


{{{x=sqrt(12^2+8^2)=sqrt(144+64)=sqrt(208)=4*sqrt(13)}}}


Assumption 3:  It is NOT a right triangle:


All you can say is that {{{4<x<20}}}, excluding the two values calculated above.


Hope this helps.  And do be a little more specific next time you ask for help.