A linear function is one whose formula can be written in the form
Ax + By = C where A, B, C are real numbers and A, B are not both 0. (This is standard form, too.)

"Cross multiply."

Subtract 12.

Subtract 2y.

This matches up with standard form with A = 3, B = -2, C = -20.
It's a linear function.
36 pairs
because 2 spots = 1 pair
so 2x=72
and your answer will be 36 pairs
His opinion was that christianity needed to be spread
This is just an example i came up with randomly:
Emma is going somewhere for vacation. Emma has been traveling for 6 hours and in those 6 hours, she has travelled 36 miles. How many miles did she travel in one hour?
Nothing changes if you don't add anything.
Example:
10+10=20
20+0=20
nothing changes.