Question 388591
S(-a, b); P(a, b); Q(a, -b); R(-a, -b)
S(-2,2);P(2,2);Q(2,-2);R(-2,-2)
a. (find missing coordinates. I when ahead and provided all coordinates.)
b. Find the length of the diagonals
S(-2,2);Q(2,-2);
Distance formula 
D= {{{sqrt((x1-x2)^2+(y1-y2)^2)}}}

D= {{{sqrt((-2-2)^2+(2-(-2))^2)}}}
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D={{{sqrt(32)}}}
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{{{(highlight(D= 5.66))}}}the diagonal length

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(-2,-2),P(2,2);
Since these pointsform  a square the other diagonal will also have same length.
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m.ananth@hotmail.ca