Question 201558
In general, if you know f(x) then to find f(<i>anything</i>) you<ol><li>replace all instances of x in f(x) with <i>anything</i> surrounded by parentheses</li><li>simplfy</li></ol><br>
For your {{{f(x) = sqrt(-9x)}}}
{{{f(y) = sqrt(-9(y)) = sqrt(-9y)}}}
{{{f(-2) = sqrt(-9(-2)) = sqrt(18)}}}
{{{f(x+1) = sqrt(-9(x+1)) = sqrt(-9x+ (-9))}}}
{{{f(3x+y) = sqrt(-9(3x+y)) = sqrt(-27x+ (-9y))}}}
and last but not least:
{{{f(-k) = sqrt(-9(-k)) = sqrt(9k)}}}