since the perimeter is 21cm
you know 2x+5+3x+1+x=21
you can simplify this to 6x + 6 = 21cm
you can then take away 6 from both sides
leaving you with 6x = 15
divide those by 6
leaving you with x = 2.5
you can then check this by doing 2.5 times 6 = 15
then 15 + 6
= 21
ANSWER- x = 2.5