We recognise the two "vertical" 2.0 ohm resistors as being parallel to each other. This system is connected to a series combination of 2.0 ohm and 3.0 ohm resistors along with the ammeter and the battery.

For the parallel branch involving the two 2.0 ohm resistors, their effective resistance is given by

1/R = 1/2.0 + 1/2.0
1/R = 1.0
R = 1.0 ohm

So, the two 2.0 ohm resistors in parallel is as good as a single 1.0 ohm resistor in terms of the resistance provided.

The total resistance of the circuit would then be 1.0 + 2.0 + 3.0 = 6.0 ohm.

So, the whole set up can be viewed as a battery, an ammeter and a 6.0 ohm resistor, and nothing else.

Ammeter reading
= Current flowing in the main circuit
= V/R
= 12/6.0
= 2.0 A