Let x be the width of the walkway.
Length of the garden + walkway = 42 + x
Width of the garden + walkway = 20 + x
Area of the garden including walkway = 1,248 ft²
Area = (L) (W)
1,248 = (42+x)(20+x)
1,248 = x² + 62x + 840
x² + 62x + 840 - 1248 = 0
x²+ 62x - 408 = 0
Factor: (x + 68) (x - 6) = 0
x + 68 = 0 x - 6 = 0
x = - 68 x = 6
Choose the positive root, x = 6
The width of the walkway is 6 ft².
To check:
(42 + 6 ft.) ( 20 + 6 ft.) = 1,248 ft²
(48 ft.) (26 ft.) = 1,248 ft²
1,248 ft² = 1,248 ft²