see diagram
look a right triangle
for a right triangle
with legs legnth a and b and hypotonuse c
a²+b²=c²
so
the legs are 7 and the diagonal is c
a=b since all 4 sides of a square are equal legnth
7²+7²=c²
49+49=c²
98=c²
sqrt both sides
√98=c
(√49)(√2)=c
(7)(√2)=c
7√2=c
the legnth of the diagonal is 7√2 inches or aprox 9.899 inches