You would have to add 57 g of solute.
I assume that your instructor is asking you to use a solubility curve like the one below to answer the question.
Note that the curve gives the solubility in terms of mass of solute per 100 g of water.
It shows that the solubility of potassium nitrate is 49 g/100 g water at 30 °C
and 140 g/100 g water at 70 °C.
You are using only 63 g of water.
At 30 °C,
At 70 °C,
Thus, to keep the solution saturated at 70 °C, you would have to add
Note: The answer can have no decimal places because we did not read the graph to any decimal places.