modular arithmetic - Solving system of linear congruences

I have the following:

$$12x+28y=20$$

I'm trying to find solutions to the equation above defined by: $12x\equiv 20\pmod {28}$

The GCD is $d = gcd(28,12)=4$ and since $4 | 20$, then there are 4 solutions that exist. (please correct me if I'm wrong).

Using the extending Euclidean Algorithm, we find $x_0=-2$ and $y_0=1$. The general solution is defined by:

$$x_0+t(\frac nd)$$ which in turn gives $-2+7t$ in our case. But how can we have a negative remainder if $x=-2 \pmod 7$ which can't happen.