Answer:
squrt6(84)
Step-by-step explanation:
By the law:
squrtn(a^m) = a^(m/n)
Best regards
since the lengths of all those four sides are in a 3:2:6:7 ratio, and the whole perimeter is 126, what we do is, simply divide the whole by (3+2+6+7) and distribute accordingly.

7 is the right answer your looking for.
3y=2(2+2y); 3y=4+4y; -y=4; y=-4;
5,6
55,56
65,66
555,556,565,566
655,656.665.666
Looks like there are 14 numbers that use only the digits 5 or 6 or both 5 or 6 that are under 1000.