Is 3x equals 5y a linear equation?

YES!!!

When neither 'x' nor 'y' are taised to any power/index/exponent. then it is a linear equation.

3x = 5y

5y = 3x

Divide both sides by '5'

y = (3/5)x + 0

In other Words it is a straight line, through the origin (0,0) of gradient '3/5'.