#x^2+100=20x#

#x^2 color(white)(aaaaaa)+100=color(white)a20x#

#color(white)(aa)-20xcolor(white)(aaaaaaa)-20xcolor(white)(aaa)#Subtract #20x# from both sides.

#x^2-20x+100=0#

#(x-10)(x-10)=0color(white)(aaaaa)#Factor

#x-10=0color(white)(aaa)x-10=0color(white)(aaa)#Set each factor equal to zero

#x-10=color(white)(aa)0#

#color(white)(a)+10color(white)(aa)+10color(white)(aaa)#Add 10 to both sides

#x=10#