It is given that x is 5 less than the quotient of y and 3.
And given value of y is 18 .
So quotient of y and 3 is

And 5 less than 6 =1
So we have y=18 and x=1
Given expression is

At x=1 and y=18, we will get


I converted everything into decimals and went from there.
Answer:
x∈{-7√2, 7√2}
Step-by-step explanation:

For the equation,
y = 1/2(x+3)^2 -5
The vertice is (-3,-5). Therefore the axis of symmetry is X = -3.