You meant "find the constant of proportionality"
Given y=2 when x= -1/2, and looking for a formula such as y = k*x, 
substitute the given value (2) for y and the given value (-1/2) for x:
2 = k * (-1/2).  Solve this for k.      Then write out the general formula y = k * x, and let x= -0.3.