One piece will be length x and the other piece will be 20 cm longer, so it will be x + 20 cm long.
Added together the length of these two boards will equal 86 cm. So you can write an equation:
x + (x + 20) = 86
Remove the parentheses and add the two x's together to get:
2x + 20 = 86
Subtract 20 from both sides:
2x = 66
Divide both sides by 2 and you have:
x = 33
The short piece is 33 cm and the other piece is 20 cm longer or 33 + 20 = 53 cm.