The diagram shows a right angle triangle..

Area of triangle = base x height x 1/2

18 = (x - 1) • (x + 4) • 1/2

18 • 2 = (x - 1) • (x + 4)

36 = (x - 1)(x + 4)

36 = (x² + 4x - 1x - 4)

36 = x² + 3x - 4

0 = x² + 3x - 4 - 36

x² + 3x - 40 = 0

(x + 8)(x - 5) = 0

x + 8 = 0 or x - 5 = 0

x = -8 or x = 5

“-8” is rejected, as length/width/height can’t be negative.

The length of the shortest side the triangle:

Short side = x - 1

= 5 - 1

= 4

The best estimate would be about 1 foot