For a function to be even, it must be symmetrical to the y-axis.
Although we have f(x) = x² + 3, let's just imagine the parent graph
f(x) = x² which has a y-intercept of 0 and is of average width.
This is symmetrical about the y-axis, but we want to know
whether or not f(x) = x² + 3 is symmetrical to the y-axis.
We can shift the parent graph f(x) = x² by adding
a constant, 3, to give us f(x) = x² + 3.
Translating the parent up or down will always result
in an even function because it will always be symmetrical.