Question

Is y=5 a linear equation? Explain

Answer 1

Yes it is.

For every value of 'x' you can name, the value of 'y' is 5 .

If you mark several of those points on a graph, you see that
they form a straight line, which is horizontal and crosses the
y-axis at the point where 'y' is 5.

If the equation were not a linear one, its graph wouldn't be a straight line.