This question is impossible to answer without another equation (making a system of equations) but here is y in terms of x:
Subtract 4.2x on both sides to get -1.4y= 2.1 - 4.2x
Then divide both sides by -1.4 to get y = (2.1- 4.2x)/-1.4
Simplify by factoring out a 0.7 to get y= 0.7((0.3-0.6x)/-0.2)