Please explain this question for me. I know that value of stock is using formula P=Div₁/r-g. Dividend is $5 but why the dividend is 3 when we are calculating the value of stock after the plow back ?

Example : MY company forecasts to pay a $500 dividend next year , which represents 100 % of its earnings . This will provide investors with a 12 9 with expected return . What if instead , MY Company decides to plow back 40 % of the earnings at the firm's current return on equity of 20% . Calculate the value of the stock before and after the plowback decision