First, multiply the number of forest trolls by 4: 6 trolls * 4 = <<6*4=24>>24 trolls
Then subtract 6 from that number to find the number of bridge trolls: 24 trolls - 6 trolls = <<24-6=18>>18 trolls
Then divide that number by 2 to find the number of plains trolls: 18 trolls / 2 = <<18/2=9>>9 trolls
Then add the number of trolls she saw in each location to find the total number: 6 trolls + 18 trolls + 9 trolls = <<6+18+9=33>>33 trolls
#### 33