Which number is in the domain of f(x)

We should first find where the graph doesn't exist, which would be where the denominator is equal to 0 or the formula under the radical is negative. So we can write the equation x-3≤0 to find where the graph can't exist
x-3≤0
x≤3

So this means that no x values less than or equal to 3 can exist on this graph. So the only choice that isn't less than or equal to 3 is choice 2 (7)

Hope this helps