Change the line
Box::Box(double 10.0,double 20.0,double 30.0);

to
Box(double l,double w,double d);

and remove the semicolons off the Box::Box, Box::volume and Box::area function bodies. (You didn't put one after main, which is correct, so it's not clear why you think you should put one after other functions.)

In a function prototype you specify variable names, not values.
Function bodies should not include a semicolon between the close bracket and open brace.