tolong yang bisa jawab, tolong juga pakai cara

a + 2b = 21 ×2
2a + b = 18 ×1
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2a + 4b = 42
2a + b. = 18
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0. + 3b = 24
b = 24÷3
B = 8

a + 2b = 21
a + 2×8 = 21
a + 16 = 21
a = 21-16
a = 5