FORECAST(x_val, known_y_vals, known_x_vals) essentially performs a linear regression on the known_y_vals and known_x_values and "predicts" the y_val for the input x_val.

For example, lets say we had known_x_vals = {1, 2, 3, 4} and known_y_vals = {2, 4, 6, 8}. It is pretty obvious that for an x_val of 5, we should have a y_val of 10.

This is what FORECAST does, except it can handle more than just linear regressions.