We cannot do crossing over.
Let's rearrange the equation
#1/4<7/(7-x)#
#7/(7-x)-1/4>0#
#(28-(7-x))/(4(7-x))>0#
#(21+x)/(4(7-x))>0#
Let #f(x)=(21+x)/(4(7-x))#
Let's build the sign chart
#color(white)(aaaa)##x##color(white)(aaaa)##-oo##color(white)(aaaa)##-21##color(white)(aaaaaaa)##7##color(white)(aaaaaaa)##+oo#
#color(white)(aaaa)##21+x##color(white)(aaaaa)##-##color(white)(aaaa)##+##color(white)(aaaa)##||##color(white)(aaaa)##+#
#color(white)(aaaa)##7-x##color(white)(aaaaaa)##-##color(white)(aaaa)##+##color(white)(aaaa)##||##color(white)(aaaa)##-#
Therefore,
#f(x)>0# when #x in (-21,7)#