Answer:
[tex]x=15[/tex]
Step-by-step explanation:
Use the Pythagorean Theorem
[tex]a^2-b^2=c^2[/tex]
With c being the hypotenuse, or x in this case
[tex]9^2+12^2=x^2[/tex]
[tex]81+144=x^2[/tex]
[tex]225=x^2[/tex]
Now take the square root of 225
[tex]\sqrt{225}=x[/tex]
[tex]\sqrt{225}=15[/tex]
[tex]x=15[/tex]
