If it's directly related, it would be y = x + b.
Given coordinates (84, -252)
-252 = 84 + b
b = -252 - 84 = -336
y = x - 336
y = 74 - 336 = -262
Hope this helps, and whoever wrote this problem seriously needs to reconsider their grammar and usage of "with" and "when" and "but" in the right context, it just seems more confusing and misused.