The maximum point on the curve with equation y = f(x + a) is on the y-axis.
(c) Write down the value of the constant a.
You can see that the maximum point on f(x) is at (2,5)
However, the question states that the maximum value of f(x+a) is on the y-axis, so we need to apply some transformation to f(x) to allow it to reach the y-axis...
Quite simply, a = 2
because f(x+2) moves f(x) 2 units to the left
and hence the maximum will be on (0,5) which is on the y axis