To solve for the value of "a", we'll need to use the Pythagorean Theorem.
Pythagorean Theorem: a^2 + b^2 = c^2
---a and b are the legs of the triangle and c is the hypotenuse
Plug in known values and simplify:
a^2 + (9)^2 = (12)^2
a^2 + 81 = 144
a^2 = 63
a = sqrt(63)
The number that should go beneath the radical symbol is 63.
Hope this helps!