EXPLANATION
Let's see the facts:
Perimeter = P = 312 m
Length = l = 89m
Width = w = unknown
The perimeter of a rectangle is given by the following relationship:

Replacing terms:

Applying the distributive property:

Subtracting 178 to both sides:


Dividing 2 to both sides:

Simplifying:

Switching sides:

The width of the garden is 67 meters.