Generally, it should look something like this.
radian = degree * pi/180; % Since your x (lng) and y (lat) are given in decimal degrees.
earth_radius = 6378.137;
lng = (x * radian - 135/180*pi) * earth_radius; %[km]
lat = atanh(sin(y)) * earth_radius; %[km]
Since you have a structures, it is better to write function mercator that your data can easily be implemented.
