It is correct to write ھذا Above the ھ is a ' symbol which gives it the "haa" sound.
As I read The Holy Quran there are symbol(s) (instead of preceding/succeeding vowels) on each letter which tell how the letter is connected to its preceding/ succeeding letter.
I don't have symbols available on my keyboard but I'll just write the sounds for you to understand InShaaALLAH
I am using the letter "h" as as example.
ha, hi, hu, haa, hii, huu, han, hin, hun, hh, ah, ih, uh
Sorry, I didn't have the option of symbols, but I hope it helps.
The word مكتب (office) comes from the root كتب (write). Other words stemming from this root are كتاب (book), كاتب (writer), or مكتبة (library).
It takes some time to get used to this kind of forming words from the root but when you know about it it helps recognising the patterns.