If the economy is normal, Charleston Freight stock is expected to return 14.3 percent. If the economy falls into a recession, the stock's return is projected at a negative 8.7 percent. The probability of a normal economy is 80 percent. What is the standard deviation of the returns on this stock?

I can get to:

= 0.80 (0.143-0.97)^2 + 0.20 (-0.087 -0.97)^2

^ I feel like I can't do this basic math and get the correct answer! PLease show me the steps!