We know that Heidi has x donations. We also know that Granpa has 2 fewer donations than Heildi, which means to subtract so x - 2. We also know that there are 7 donations in all. This tells use donations = 7 in all. So
Heidi donations = x
Grandpa = x - 2
Heidi + Grandpa = Total donations
x + (x-2) = 7