Answer
$(I\cup W)\cap E^{\prime}$
Work Step by Step
"... but you will not..." has the same meaning as
"... and you will not..."
Restructure:
Either your math grade will improve
OR
you will use the Website tonight,
AND
you will not use it every night.
This describes the event (I or W) and (not E)
Symbollically: $(I\cup W)\cap E^{\prime}$