Nomor 15 caranya bagaimana.

f(x) = a-bx

f(-1) = a-b(-1)
-5 = a+b

f(2) = a-b(2)
13 = a-2b

trs disubtitusi
a+b = -5
a-2b = 13
---------------- -
3b = - 18
b = -6

a+b = -5
a - 6 = -5
a = -5+6
a = 1

f(-5) = 1-(-6)(-5)
f(-5) = 1 - 30
= -29 (C)


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