The best answer is a, 20.78 units. You can use the Pythagorean theorem to solve this: a^2+b^2=c^2. Just plug in the variables and solve it as a multi step equation. The legs of the right triangle are a and b, which in this case you would plug in those variables with x and 12. The hypotenuse is c, which in this case is 24. That will leave you with 12^2+b^2=24^2. Simplify those exponents and continue solving for b. At the last part of your equation solving, you’ll be left with b^2= a number. To find the value of b just find the s quake root of that number and there’s your answer.