The points (3, 24) and (7, 56) represent points of a function where y, the number of photographs, varies directly with x, the number of pages in an album. which statement describes another point on the graph of this function

It says that "a function where y, the number of photographs, varies directly with x, the number of pages in an album"

It has the form of "y=kx"
k=8 from the two points given, so the function is y=8x 
The first answer 400=8(50) is the only statement that fits the function. 

The correct answer is this one: 50-page photo album holds 400 photograph