Partial Solve?

I am a noob to hashcat, and modern crypto, and I was wondering if theres a way to solve an encrypted string for say, just the first few characters, even if the bulk of the string is unknown.

ie:
xxyyyyyyyyyyyyyyyyyyyzzzzzzzzzzzzzzzzzzzzzzzzzzzzz

I know what y is, I dont know what z or x is,BUT I only need x.