@vijju
what is the max and min value for this term [g(n)*h(n)] , min would be n as g(n) cant go below n and h(n) can become constant that case would give minimum value,
now think what can be its max value,you cant predict it
so for term [g(n)*h(n)] what you can surely say is its minimum value would be n, i.e omega(n)
now total exprsn is f(n)+[g(n)*h(n)], f(n) is theta(n) , it cant go beyond n or less than n ,so what you can say from total exprsn that value of this exprsn will always be greater or equal to n i.e omega(n)