(10+×)^2
expand
(10+x)(10+x)
multiply
100+10x+10x+x^2
add
100+20x+x^2
rearrange
x^2+20x+100
if this is what you mean this is the answer if you want to know what x is just set ()=0 and solve
x+10=0
x=-10
but I believe you are asking for the equation hope that helps let me know :)