Question 214172
Let the length of a side of the square = s, then the perimeter is expressed by:
{{{P = 2(s+s)}}} Increase one side by 8cm. to get (s+8)cm. and decrease an adjacent side by 2cm. to get (s-2)cm.. The resulting perimeter (P) is 40cm. 
{{{40 = 2((s+8)+(s-2))}}} Divide both sides by 2.
{{{20 = s+8+s-2}}} Simplify.
{{{20 = 2s+6}}} Subtract 6 from both sdes.
{{{14 = 2s}}} Divide both sides by 2.
{{{7  = s}}}
The length of a side of the square is 7cm.