Perimeter is 2l + 2w (l=length w=width) so if l=3+2w and you plug that in you get 2(3+2w)+2w=perimeter.
Distribute to simplify equation
P=6+4w+2w
Then simplify like terms
P=6+6w
Then plug perimeter in
128=6+6w
Solve
122=6w
W=20.333
And plug that in the original equation for length
128=2l+2(20.333)
128=2l+40.666
2l=87.3333
L=43.666