Question 1038558
Let {{{ w }}} = width of garden in meters
{{{ 3w + 12 }}} = length in meters
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{{{ w*( 3w + 12 ) = 180 }}}
{{{ 3w^2 + 12w = 180 }}}
{{{ w^2 + 4w - 60 = 0 }}}
{{{ ( w + 10 )*( w - 6 ) = 0 }}} ( by inspection )
{{{ w = -10 }}} ( can't use -width must be positive )
{{{ w = 6 }}} 
and
{{{ 3w + 12 = 3*6 + 12 }}}
{{{ 3w + 12 = 30 }}}
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The dimensions are: 6 m  by 30 m
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check:
{{{ 6*30 = 180 }}} m2
OK