The Sassanid empire was the last kingdom of Persia before the Islam began.
General Khalid ibn Walid led the Arab muslims to attack the Sassanid empire first in the year 633.
The second attack was carried out by Sa'd ibn Abi Waqqas leading a victory for the Arab Muslims at the Battle of al-Qadisiyyah in the year 636 . The Sassanid empire lost control of West of Iran.
The final invasion was carried out by Caliph Umar and the fall of the Sassanid empire to the Arab Muslims was complete in year 651.
The lines that tell you the legs are the same length let you know that angle a and angle acb are the same. so 180 -30 is 150 divided by 2 is 75 so c is 75. then take 180 -75 and you get x.
