How to reference external subprojects in gradle

I have a sample project with the following setup:

• Root project A
  • Sub Project 1
  • Sub Project 2
• Root project B
  • Sub Project 1
  • Sub Project 2

I add the following code in subproject settings A-1. gradle for link from subproject B-1:

include ':RootProjectB:SubProject1'
project(':RootProjectB:SubProject1').projectDir = new File('/Users/RootProjectB/SubProject1')

      

        
        
        
      

    

but subproject B-1 needs root B config project and get compile error.
so how to reference from subproject B-1?