$P(A/B) = \frac{P(A \cap B)}{P(B)}$
Where $P(A/B)$ = Probability of A after B has happened.
$P(A \cap B)$ = probability of happening A and B together.
$P(A)$ = Probability of happening event A.( Event : In a toss no, of heads is = 2 )
$P(B)$ = Probability of happening event B. ( Event : First toss is head )
In this questions, $P(A \cap B)$ = 1/4 ( HTH and HHT out of 8 possible cases)
and $P(B)$ = 1/2 ( HTT,HTH,HHT,HHH out of all 8 possible cases )
Here we can count manually since no. of tosses are less. For large case we need to use combination formule to make counting simple