Can you explain 2x-y=0, what is it and how to solve it ?

The equation you supplied is called a simultaneous equation.
This means that in order to find the answer you will need eliminate elements on both sides of the equation equally.
so
please note that anything times 0 or divide by 0 is equal to 0
2x - y =0

so
0= 2x -y
y = 2x
x= y/2