The Hebrew letters for strength have changed over time.
These symbols are called 'Al' or 'Aleph', and has the meaning of strength or power. As you can see from the picture, the symbol has developed over time bringing it to the symbol that is used today.
The primary symbol looks like an ox's head, as these creatures are renown for being strong, especially shown by the work that they used to do for people.
The name of the symbol 'Aleph' has links with the Greek 'Alpha', which also means leader and strength etc.
The way to write strength in Hebrew would be like this: Strength = ko'ach (כוח). The symbols in the brackets is how the word would be written in Hebrew and the word before the bracket is how the word is said.