Prove that ( There exist exactly seven lines ) that in geomatry fano system please help thank..

35=5×77=7×1when we simplify 7 by 7we will get 5 and 1, 5 divided by 1 is 5

Use pythagorean’s theorem and find the missing side lengths of the two smaller triangles and add them together
13^2 - 12^2 = 5
15^2 - 12^2 = 9
5+9 = 14
14 is the length of each side of the square
14^2 + 14^2 = 19.8
19.8 is the diagonal length of the inside if the square