Question 873264
A rectangle's diagonal is a hypotenuse of a right triangle.

Therefore given a width (a)which is 12 in and a hypotenuse (c) which is 13 in, the length (b) will be:

{{{b^2=c^2-a^2}}}
{{{b^2=(13)^2-(12)^2}}}
{{{b^2=169-144}}}
{{{b^2=25}}}
{{{b=5}}}

The area:

{{{A=(ab)/2}}}
{{{A=(12*5)/2}}}
{{{A=30in^2}}}
Hope that helps!
Steffy