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f(x) = ax+b
f(-2) => -2a+b=8 ......(1)
f(5) => 5a+b = -13...... (2)

Eliminasi pers (1) dan (2)
-2a+b =8
5a+b=-13 (-)
=> -7a = 21
a = -3

Sub per (1)
-2a+b =8
-2(-3)+b=8
6+b=8
b=2

Maka nilai a = -3 dan b = 2