Explanation
The first step is to draw a representation of the given parameters.
Since this is a right-angle triangle, the Pythagorean theorem applies. This can be seen below;
[tex]\text{Longest Leg}^2=sum\text{ of the square of the short legs}[/tex]
We can then apply it to the sides of the triangle.
DF is the longest side. Therefore,
Answer
[tex]DF^2=DE^2+EF^2[/tex]
