sin(45degrees) or BC/AB is 1/sqrt2(that's the value of sin45 degrees I recommend you to memorize this.)
x/12=1/sqrt2
x*sqrt2=12
x=12/sqrt2
Now rationalize this fraction to make the answer more formal.
x=12sqrt2/2(I multiplied both top and bottom by sqrt2)
Now simplify
x=6sqrt2
Question done! answer is C 6 square root of 2
Hope this helped any questions just put them in the comments.
