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Let x be the width
     x+8 be the length

A = w·l

Substitute the given,
105 = x (x+8)
105 = x² + 8x
0 = x² + 8x -105
0 = (x+15) (x-7)
x = -15  ; x = 7       [Use x=7 since there is no negative dimension]

Width = 7m
Length = 15m         [from x+8]
Thus, we have the equation x(x+8)=105. This is similar to length times width equals area. it :)
Have you learned about the factoring part?
Okay :)