Suku ke 5 barisan aritmatika 19 dan suku ke 11 sama dengan 43, suku ke 15? Bagaiman cara menghitungbya?

b=Um-Un/(m-n)

b=(U11-U5)/(11-5)
b=(43-19)/6
b=24/6
b=4

U5=19
a+4b=19
a+4(4)=19
a+16=19
a=19-16
a=3

U15=a+14b
U15=3+14(4)
U15=3+56
U15=59

U5 = 19
a + 4b = 19.......(1)

U11 = 43
a + 10b = 43.....(2)

(1) & (2)
a + 4b = 19
a + 10b = 43
------------------- -
-6b = -24
b = -24/-6
b = 4

b = 4 subtitusi ke (1)
a + 4b = 19
a + 4 . 4 = 19
a + 16 = 19
a = 19-16
a = 3

U15 = a + 14b = 3 + 14 . 4 = 3 + 56 = 59