What is the equation of a hyperbola with a = 9 and c = 12? assume that the transverse axis is horizontal

Given a and c, we solve for b by using the formula: 
c² = a² + b²
12² = 9² + b²
b = 3√7

The standard equation for a hyperbola with a horizontal transverse axis is  
(x-h)²/a² - (y-k)²/b² = 1, where (h,k) is the center.

Assuming that center is (0,0), the equation of the hyperbola will be:

x²/9² - y²/(3√7)² = 1 or
x²/81 - y²/63 = 1