I guess, 勾股幂合, 以成弦幂. 勾的幂与股的幂合并，就成为弦的幂。
朱幂 and 黄幂 are colored areas (red and yellow) in the figure. I think the reprint of the book should be in color so that people can understand better.
幂 is the product of two numbers, which also means the area of a square or rectangle.