Linear equation expressions (SAT/PSAT 8/9/10). Game on.

Subtract $6$ from both sides: $3x = 18$, so $x = 6$. Then $2x = 2 \cdot 6 = 12\textsf{.}$

Subtract $6$ from both sides: $2x = 20$, so $x = 10$. Then $15 - x = 15 - 10 = 5\textsf{.}$

Divide both sides by $6$: $x = 9$. Then $x + 7 = 9 + 7 = 16\textsf{.}$

Subtract $8$ from both sides: $2x = 14$, so $x = 7$. Then $3x = 3 \cdot 7 = 21\textsf{.}$

Add $3$ to both sides: $9x = 45$, so $x = 5$. Then $x - 1 = 5 - 1 = 4\textsf{.}$

Subtract $4$ from both sides: $8x = 32$, so $x = 4$. Then $2x = 2 \cdot 4 = 8\textsf{.}$

Divide both sides by $12$: $x = 5$. Then $5x = 5 \cdot 5 = 25\textsf{.}$

Add $15$ to both sides: $3x = 15$, so $x = 5$. Then $x + 4 = 5 + 4 = 9\textsf{.}$