Computer Programming "While Loops" lesson

I got most of it down, and it shows. But when I get to coding Y, the numbers are just 400 going down when the x numbers going across are 0,50,100,150,200, and so forth. I don't know what to do to change it. Help please? I'll type out my coding here. 

fill(17, 0, 255);

// a handy dandy ruler across the top
var x = 0;
var y = 0;
while (x < 400){ text(x, x, 10); x+=50;}

while (y < 400){ text(x, 0, y); y+=50;}

What am I doing wrong? The error says "