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