perimeter= 2L + 2w
where L is the length and w is the width
given the perimeter and the width, we can now calculate the length
140 m = 2L + 2 (30 m)
140 m = 2 L + 60 m
isolate the variable L to one side of the equation by transposing the constant to the other side. we get:
140 m - 60 m = 2L
80 m = 2L
divide both side by 2 to get L
40 m = L
Length is equal to 40 meters