Question 53532
The opposite sides of rectangles are equal.  You have 2 widths (W)of the same size and 2 lengths (L) of the same size.  If you add up all the sides of a figure, you get its perimater (P).  Therefore, the formula for the perimeter of a rectangle is:
P=W+W+L+L
P=2W+2L
P=34 cm
L=x
W=x-7
Substitute these values into the formula for the perimeter of a rectangle and you get: 
34=2(x-7)+2x
34=2(x)+2(-7)+2x
34=2x-14+2x
34=(2+2)x-14
34=4x-14
34+14=4x-14+14
48=4x
48/4=4x/4
12=x
Length=x=12 cm
Width=x-7=12-7=5 cm