The graph of a sinusoidal function intersects its midline at (0,-6)(0,−6)left parenthesis, 0, comma, minus, 6, right parenthesis and then has a minimum point at (2.5,-9)(2.5,−9)left parenthesis, 2, point, 5, comma, minus, 9, right parenthesis.
Write the formula of the function, where xxx is entered in radians.