Problem 1
Diego could have squared either -2 or 2
(-2)^2 = (-2)*(-2) = 4
(2)^2 = (2)*(2) = 4
Andre could have squared whatever Diego didn't square. If Diego picks -2, then Andre would pick 2, or vice versa.
================================================
Problem 2
If you haven't learned about complex or imaginary numbers yet, then the answer would be "There's no such number".
However, if your teacher has covered complex or imaginary numbers by this point, then Jada could pick either -2i or 2i
(-2i)^2 = (-2i)*(-2i) = 4i^2 = 4(-1) = -4
(2i)^2 = (2i)*(2i) = 4i^2 = 4(-1) = -4
Whatever Jada picks, Elena will pick the opposite value.