Answer:
The percent (relative) change is 7.14 percent (%).
Explanation:
We are given that the original support value is at 42%.
Then the support increased by 3%, to reach 45%.
We want the Percentage Change (i.e. relative) change of this increase, which can be computed as:

where
is the final value reached
is the original value
So plugging in our known values and computing we have:
percent increase.
Now lets go back and check if we are correct.

So it works!