Answer:
D. [tex]x=10[/tex]
Step-by-step explanation:
You can use the Pythagorean Theorem, [tex]a^2+b^2=c^2[/tex], where a and b are the legs and c is the hypotenuse.
[tex]12^2+b^2=13^2\\144+b^2=169[/tex]
Subtract 144 from both sides
[tex]b^2=25[/tex]
Square root both sides
[tex]b=5[/tex]
[tex]2b=x\\b=5\\x=10[/tex]
