Answer: 10.6 mm
Step-by-step explanation:
Perimeter (P) is the sum of the sides. Â
The trapezoid has 4 sides: the shorter base = 1.4mm
                       the longer base = 16.6mm
                       the left side  = x
                       the right side = x  Â
P = short base + long base + left side + right side
40.8 = 1.4 + 16.6 + x + x
40.8 = 18 + 2x
21.2 = 2x
10.6 = x