PASCAL 2012 Problem 13

The operation \nabla is defined by g\nabla h=g^2-h^2. For example, 2\nabla1=2^2-1^2=3. If g>0 and g\nabla6=45, the value of g is

Answer Choices
A. 39
B. 6
C. 81
D. 3
E. 9