Answer : The number of larvae is greatest at 5 degrees Celsius.
Given the equation 
x is the temperature of water in degrees Celsius, and y is the number of insect larvae in the water
Peak of the graph is the vertex
Now we find out the vertex from the given equation
To find x coordinate of vertex we use formula x= 
a= -2, b= 20
So x= 
x = 
x = 5
Now we find out y
Plug in 5 in the place of x

y= 450
Vertex is (5,450)
It means the number of larvae reaches the maximum of 450 at 5 degree Celsius
Hence , the number of larvae is greatest at 5 degrees Celsius.