If you have an equation y = f(x) then you can define x=g(t), in which case substituting into the equation for y, you get y=f(g(t)). The only constraint on the choice on the function g(t) is that the range of g(t) includes the entire domain of f(x). That way the graph of the parametric equations is the entire graph, not a partial one, of the original equation.
In your case, the two answers you gave are correct, as are many others: y = 1/x3 = 1/(2t)3 = 1/(8t3) in the first pair of equations, and y = 1/x3 = 1/t3 in the second pair of equations.
In the future, don't be afraid if you are not sure of an answer. Even if you were to have it wrong, teachers and tutors are here to help you learn from mistakes, not criticise your abilities/intelligence. In our minds, you're a smart person.