Question

(x+10)^2=(x+9)^2

Answer 1

first expand:
x^2+20x+100=x^2+18x+81
collect like terms:
2x=-19
x=-19/2

Answer 2

You can also do (x+10) = -(x+9) and get the same answer.
Since, x+10 can never be equal to x+9.