['Step 1: Find the mean of x, and the mean of y.', 'Step 2: Subtract the mean of x from every x value (call them "a"), and subtract the mean of y from every y value (call them "b")', 'Step 3: Calculate: ab, a2 and b2 for every value.', 'Step 4: Sum up ab, sum up a2 and sum up b.']