Your string number 97.9
is being converted to double, but in your current computer culture the points act as a thousands separator, and commas as decimals separator, and looks like you want the point to act as a decimal separator. Is a culture problem.
Try this:
str = "97.9"
dbl = Double.Parse(str, Globalization.CultureInfo.InvariantCulture)
Take a look at the MSDN Documentation.
By the way, you should use Option Strict On
to prevent implicit conversions of types in your code.