Complexity of Context Sensitive Languages
When the book says


  Following the analysis leading to Equation (5.2),


it implies that if you apply the same kind of reasoning that resulted in Equation (5.2) to context-sensitive languages, then you will get the similar equation


  $$ N = |P| + |P|^2 + \ldots |P|^{cn} = O(|P|^{cn+1}). $$


This equation is the analog of Equation (5.2) for context-sensitive languages. To prove this equation, you use reasoning similar to that used to prove Equation (5.2).