$\"\"$

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The equations are

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$\"\"$ (Equation (1)

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$\"\"$ (Equation (2)

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Solve the equation 1 for variable a in terms of the variable

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$\"\"$

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$\"\"$ (Add b to each side)

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$\"\"$ (Apply additive inverse property: $\"\"$ )

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$\"\"$

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Substitute $\"\"$ for a in equation 2.

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$\"\"$

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$\"\"$ (Distribute - 2)

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$\"\"$ (Add: $\"\"$ )

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$\"\"$ (Add 4 to each side)

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Rearrange the terms using commutative property of addition: a + b = b + a.

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$\"\"$

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$\"\"$ (Add: $\"\"$ )

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$\"\"$

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Substitute 7 for b in equation (1).

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$\"\"$

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$\"\"$ (Add 7 to each side).

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$\"\"$ (Add: $\"\"$ ).

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The solution is $\"\"$

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$\"\"$

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The solution is $\"\"$ .

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