How do I get linest and intercept to ignore blank cells?

I am trying to create a very basic estimation project cost sheets, so that we can have an idea of how much a certain project will cost before we start. For example:
Project A: Roof cost 100, insulation cost 200, windows cost 50 Project B: Didn't have a roof, insulation cost 150, windows cost 30 Project C: Roof cost 70, insulation cost 150, didn't have windows
When doing linest and intercept it includes the blanks which gives off a wrong figure, how do I get it to ignore the blanks with out changing each formula individually.
Here's a small screenshot to show my problem, at the moment my formulas look like =LINEST(F3:F14,$B$3:$B$14)
SQI3IjM.png
The blue lines are the linest and intercept lines

Thank you