Question 691372
Let the original side of the square be {{{ x }}}, then new side is {{{ x+3 }}}

New Area = 25 so {{{ (x+3)(x+3)=25 }}}
{{{ x^2+6x+9=25 }}}
{{{ x^2+6x-16=0 }}} (this is a quadratic ...)
{{{ (x+8)(x-2) }}} (... which factorises nicely)
so {{{ x=-8 }}} or {{{ x=2 }}}

But x is the side of a rectangle, so {{{ x >0 }}}
Hence, solution is {{{x=2}}}