Easy and hard are at opposite ends of the scale. What is in the middle?
“This is easy.”
“This is ___.” <— what goes there?
“This is hard.”
Solution 1:
Moderately difficult - would be acceptable but I don't think there is a good single word answer.
Solution 2:
A few options here - but there really isn't a middle ground that is as universally understood as easy/hard.
This is possible.
This is straightforward.
This is achievable.