If three resistors 2ohm, 3ohm and 4ohm are connected in a circuit. Calculate the equivalent resistance of the combination

You haven't described whether they're connected in series or in parallel.  Actually, there are eight (8) different ways they can be arranged, and each way has a different equivalent resistance.

==> All 3 resistors in series.  Equivalent resistance = (2+3+4) = 9.000 ohms

==> All 3 resistors in parallel.

Equivalent resistance = 1 / (1/2 + 1/3 + 1/4) = (12/13) ohm or 0.923 ohm

==> The 2 ohm resistor in series with (the 3 and the 4 in parallel)

Equivalent resistance = 2 + 1/(1/3 + 1/4) =  3.714 ohms

==> The 3 ohm resistor in series with (the 2 and the 4 in parallel)

Equivalent resistance = 3 + 1/(1/2 + 1/4) =  4.333 ohms  

==> The 4 ohm resistor in series with (the 2 and the 3 in parallel)

Equivalent resistance = 4 + 1/(1/2 + 1/3) =  5.200 ohms

==> The 2 ohm resistor in parallel with (the 3 and the 4 in series)

Equivalent resistance = 1.556 ohms

==> The 3 ohm resistor in parallel with (the 2 and the 4 in series)

Equivalent resistance = 2.000 ohms

==> The 4 ohm resistor in parallel with (the 2 and the 3 in series)

Equivalent resistance = 2.222 ohms

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