Question 451602
Area, A, of this sail is 


{{{A = ah/2}}}


where a is a base and h is height of the sail.


Since height is 7 feet more than the base it comes to


{{{h = a + 7}}}


So area is now


{{{ 30 = a(a+7)/2 }}}     , multiply to get rid of parentheses

{{{ 30 = (a^2 + 7a)/2 }}} , multiply both sides by 2

{{{ 60 = a^2 + 7a }}}     , subtract 60 from both sides

{{{ a^2 + 7a - 60 = 0 }}} , this is quadratic equation

{{{x = (-7 +- sqrt( 7^2-4*1*(-60) ))/(2*1) }}}

We get a=5 or a=-12 and only acceptable solution is a=5 feet.

Height of the seal is 

h=a+7
h=5+7
h=12 feet