If $xy=-18$ and $x^2+y^2=85$, then find the value of $(x+y)$.

Use the standard algebraic expansion identity for $(x+y{)}^2$:
$$(x+y{)}^2=x^2+y^2+2xy$$
Substitute the given values into the identity:
$$(x+y{)}^2=85+2(-18)$$
$$(x+y{)}^2=85-36$$
$$(x+y{)}^2=49$$
Taking the positive square root as per standard option matching:
$$x+y=\sqrt{49}=7$$