[Rodamala]

“X + Y = 10. Solve for X.”

Subtract Y from both sides. X=10-Y

The "solution" is valid for all values of X and Y, it does take the form of a line.

y=mx+b

[ETL]

X + Y - 10

I believe he meant it as an example of a problem without a specific solution. He probably wasn’t expecting an answer.