Combine different formula

This is my problem i dont know h0)ow to combined different formula.
This is my example

item width height lenght area
1
2
3
4
5


this is the formula of item that will show in the area
staight=width+height*lenght
elbow=width+height*(lengh+150)
straight=(width+height*lenght)+(width*height)

so how to combined this formulas so that when i type the either of name of item it will use the formula of that name and it
will show automatically in the area.

Hope you understand my explanation and i hope you can help me in my problem

thank you very much