Высота равнобедренного треугольника равна 15 см , а основа - 16 см.Найдите боковую сторону треугольника

Задание : Choose the correct preposition. (если можно с объяснениями)
Текст :
Hi. My name is Diana. I am 15 years old and i am (1) in / at the 9th grade. I go (2) to / in  Blackforest Middle School. The person i admire (3) with / - is my mother. Her name is Maria and she is 36 years old. The reason i admire my mum is because she has gone (4) through / over a lot in her life. First, my mother treats bac. My mother cares (6) for / with other people especially the elderly. Secondly, i would like to say that my mother works very hard to keep a roof (7) over / about our heads. My mother gives us good values and teaches us right (8) with / from wrong. Finally i am proud (9) with / of my mother because she is brave and full (10) of / with confidence. 

Заранее спасибо:)