Let \( A \) be the point \((3, 0)\) and circles with variable diameter \( AB \) touch the circle \( x^2 + y^2 = 36 \) internally. Let the curve \( C \) be the locus of the point \( B \). If the eccentricity of \( C \) is \( e \), then \( 72e^2 \) is equal to __________.
