to find the GCF first list down the prime factors of each number then multiply the factors both have in common, or if the numbers have only one prime factor in common then that's the answer. to find the prime factors of a number just divide the number by prime factors in order.
in this question prime factors of the numbers are,
18=2×2×3
45=3×3×3
90=2×3×3×5
the common factors are 3 and 3. so now multiply 3 by 3= 9
answer=9
hope this helps....