length = x, width = y
x = 2y - 10....so sub in 2y - 10 for y back in the other equation
2x + 2y = 88
2(2y - 10) + 2y = 88
4y - 20 + 2y = 88
4y + 2y = 88 + 20
6y = 108
y = 108/6
y = 18
x = 2y - 10
x = 2(18) - 10
x = 36 - 10
x = 26
so the length (x) is 26 mm and the width (y) is 18 mm
The width of the stamp is 8 mm less then the length (26 - 18 = 8)