Tolong dijawab!! Beserta penjelasan!!

U8 = a + 7b = 43
U3 = a + 2b = 18
------------------------ -
=> 5b = 25 => b = 5
Mencari a => a + 2b = 18 => a = 18 - 10 => a= 8
S15 = 1/2.15 (2.8+14.5)
S15 = 1/2.15 (16+70)
S15 = 1/2.15 (86)
S15 = 15 (43)
S15 = 645