If resistance R is connected to a source with an EMF of 12 V, then the current strength

If resistance R is connected to a source with an EMF of 12 V, then the current strength will be 3 A, and when the resistance 2R is connected, the current will be equal to -2 A. Determine the internal resistance of the source and the value of R.

Given:
ε = 12V,
I1 = 3A,
I2 = 2A;
To find:
R -?
r -?
Let’s write Ohm’s law for the general circuit.
In the first case:
I1 = ε / (R + r);
In the second case:
I2 = ε / (2 * R + r);
From here we get:
R + r = ε / I1 = 12V / 3A = 4 Ohm;
2 * R + r = ε / I2 = 12V / 2A = 6 Ohm;
From the difference between the two equations we find:
R = 2 ohms;
r = 2 ohms.