for this problem the steps are below
recall pythagorean theorem
[tex]a {}^{2} + b {}^{2} = c {}^{2} [/tex]
where and and b are the legs of the right triangle and c is the hypotenuse
[tex]4 {}^{2} + 4 {}^{2} = d {}^{2} [/tex]
apply Pythagorean theorem
[tex] \sqrt{16 + 16 } = d[/tex]
square each 4 and take the square root of both sides
[tex] \sqrt{32} = d[/tex]
simplify
reduce the radical
FINAL ANSWER
[tex]4 \sqrt{2} = d[/tex]
or you can simply see that it's a 45 45 90 triangle and know that the hypotenuse d equal one of the legs times the square root of 2
