Choose the best definition for the following term: substitution

The best definition for the term substitution would be...

the action of replacing someone or something with another person or thing.

The word “ substitution " means that to use something new or different instead of something else. Also, it can use in math,too. For example,
I have two slope intercept form

y-2x=6

-y-4x=24

Now, we just need to substitute the two of them to find x and y

-y-4x=24
+
y-2x=6
=
-6x=30
x=-5
Then, replace variable if x in one of them to find y

y-2x=6
y-2(-5)=6
y+10=6
y=6-10
y=-4. Hope it help!