ok, so since they arre all the same length....
18. sides are all same length
so
5-2x=-4x+9
solve
add 4x both sides
5+2x=9
mins 5 both sides
2x=4
x=2
sub back
5-2x=5-2(2)=5-4=1
the length of each side is 1
3 sides so the perimiter is 3 units
19.
same side length so
3(x-1)=5x-6
distribute
3x-3=5x-6
minus 3x both sides
-3=2x-6
add 6 both sides
3=2x
divide 2
3/2=x
sub back
5x-6
5(3/2)-6
15/2-6
7.5-6
1.5
5 sides so
1.5 times 5=7.5 units is the perimiter
which is about 15/2 units
20.
we can't see what's after the x+? so this is a mystery quesiton