Can someone explain to me what the "subtraction property of equality" is?

i know what the transitive property of equality is: if a = b, and b = c, then a = c.

so for example i have a problem i'm working on where i'm proving vertical angles are congruent. i've already narrowed everything down to transitive property of equality: angle 1 + angle 4 = angle 3 + angle 4. using the subtraction property of equality will be my final step in proving congruency, but i don't know what or how to use this property. ?