Como resolver y=1-2.5

I don't really speak Spanish but I'll guess that says

How do I solve y = 1 - 2.5

Unless I'm missing something, there's not even any algebra here.

The way we subtract a bigger number from a smaller number is first we subtract the smaller number from the bigger number the regular way, then we prepend a minus sign.

y = 1 - 2.5 = - (2.5 - 1) = - 1.5