A(-1;-1) ∈ Gf <=> f(-1) = -1
f(-1) = -1 · 2 + 1 = -2 + 1 = -1 ==> -1 = -1 deci A ∈ Gf
B(-2;3) ∈ Gf <=> f(-2) = 3
f(-2) = -2 · 2 + 1 = -4 + 1 = -3 ==> -3 = -3 deci B ∈ Gf
C(1;3) ∈ Gf <=> f(1) = 3
f(1) = 2·1 + 1 = 2 + 1 = 3 ==> 3 = 3 deci C ∈ Gf
D(2;5) ∈ Gf <=> f(2) = 5
f(2) = 2 · 2 + 1 = 4 + 1 = 5 ==> 5 = 5 deci D ∈ Gf
E(0;2) ∈ Gf <=> f(0) = 2
f(0) = 2 · 0 + 1 = 0 + 1 = 1 ==> 2 ≠ 1 deci E ∉ Gf