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Answer 1

$mean = \frac{sum \: of \: the \: observations \: }{number \: of \: the \: observations}$

$the \: mean \: of \: x1 \: and \: x2 \: is \: 6$

$the \: mean \: of \: x1 \: \: x2 \: \: \: and \: x3 \: is \: 7$

$\frac{x1 \: + x2}{2} = 6$

$x1 + x2 = 2 \times 6$

$x1 + x2 = 12...........1$

$\frac{x1 + x2 + x3}{3} = 7$

$x1 + x2 + x3 = 7 \times 3$

$x1 + x2 + x3 = 21.............2$

$subtract \: \: 1 \: and \: 2 \: \: we \: get$

$(x1 + x2 + x3) - (x1 + x2) = 21 - 12$

$x3 = 9$

HOPE IT HELPS YOU