Answer GCF = 18
Step by step
List the prime factors that are common to each of the original numbers. Include the highest number of occurrences of each prime factor that is common to each original number. Multiply these together to get the GCF.
18 = 2 x 3 x 3
90 = 2 x 3 x 3 x 5
2, 3, 3 are our common factors, multiply to get 18
Attached is the tree method I use to break down each number