For Restoring the DB instance in EC2

Go to Sanpshot tab in ec2 Instances section
Copy snapshots ({500GB snapshot ID} and {150GB snapshot ID}) to us-east-1 with the aws/ebs master key. This will create a new snapshot in the aws account with the master key.

From the newly copied snapshot we need to restore the snapshot as an AMI.
Click the new sanpshot created off of {150GB snapshot ID} (150GB) and click Actions tab and click “Create Image”

On the new screen, choose the following values 
Architecture: x86_64
Virtualization type: hardware-assisted-virtualization
Root device name: /dev/sda1
Kernel ID: use default
RAM disk ID: use default

Then click add new volume and add the other sanpshot in there with following values
Volume Type: EBS
Device: /dev/sdb
Snapshot: {newly copied snapshot ID (500GB)}
Size: 500GB

Then click create.

Once created, resotre the Image as an ec2 instance.