A business plan is a formal statement of a set of business goals, the reasons they are believed attainable, and the plan for reaching those goals.

句子主干：

A business plan is a formal statement of  A, B, and C.

A,B,C分别是句中的a set of business goals，the reasons they are believed attainable，和the plan for reaching those goals

其中the reasons they are believed attainable，即the reasons [why] they are believed attainable