Question 1115409
The area of an equilateral triangle is related to its side-length, s,  by:  {{{ A =  (sqrt(3)/4)*s^2 }}}

The value of s = 54cm/3 = 18cm.  Plug in 18 to find {{{ A = 81*sqrt(3) cm^2 }}}  or about {{{140.296cm^2}}}

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EDITED to actually square s, sorry about that.