Let's solve this problem step by step.
FIRST. <em>Quadrilateral A′B′C′D′ is obtained by a reflection across the y-axis of quadrilateral ABCD</em>
When we reflect a point across the y-axis, the y-coordinate remains the same, but the x-coordinate changes in sign. So:
(x, y) → (-x, y)
SECOND. <em>Followed by a translation 3 units to the left and 3 units down.</em>
To translate a point <em>3 units to the left</em> subtract 3 from the x-coordinate and to translate a point <em>3 units down </em>subtract 3 from the y-coordinate. Therefore, the function we are looking for is:
f(x, y) = (-x - 3, y - 3)