You can use the =FORECAST(value, data_Y, data_X) formula.
value is the known value for which you want to predict the corresponding forecast (in your case B5);
data_Y is the series of data points for which you want to predict the future value (C2:C4);
data_X is the series of corresponding data points which form the basis of the forecast (B2:B4).
So in your example the formula you would put in C5 is =forecast(B5,C2:C4,B2:B4) which will return 28.7
You can find all formulas and their explanations for Google Spreadsheet at https://support.google.com/drive/bin/static.py?hl=en&topic=25273&page=table.cs