I'm assuming you're being taught how to work with negative intervals in algebra? Don't worry, that's why you're practising with temperature. Instead of asking online, try using the progress your teacher taught you. If you have notes, look back at them. Practise makes perfect :)
The total number of plants are: 12 tomatoes + 5 cucumbers = 17 plants The ratio of cucumbers plants to total plants is: 5 cucumbers to 17 plants total So It can be expressed as: 5/17 or 5:17