Question 195605
Let's make "x" the unknown side.



{{{a^2+b^2=c^2}}} Start with the Pythagorean theorem.



{{{12^2+18^2=x^2}}} Plug in {{{a=12}}}, {{{b=18}}}, {{{c=x}}} 



{{{144+18^2=x^2}}} Square {{{12}}} to get {{{144}}}.



{{{144+324=x^2}}} Square {{{18}}} to get {{{324}}}.



{{{468=x^2}}} Combine like terms.



{{{x^2=468}}} Rearrange the equation.



{{{x=sqrt(468)}}} Take the square root of both sides. Note: only the positive square root is considered (since a negative length doesn't make sense).



{{{x=6*sqrt(13)}}} Simplify the square root.



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Answer:



So the solution is {{{x=6*sqrt(13)}}} which approximates to {{{x=21.633}}}.