The equations are

                                   Equation (1)

                           Equation (2)

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

Apply addition property of equality; if a = b, then a + c = b + c.

                     (Add b to each side)

                                  (Apply additive inverse property: )

Substitute for a in equation 2.

Distribute terms using distributive property: a(b + c) = ab + ac.

                (Multiply: )

                          (Add: )

Apply addition property of equality; if a = b, then a + c = b + c.

              (Add 4 to each side)

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

b = 3 + 4                             (Apply additive inverse property: )

                                   (Add: )

Substitute 7 for b in equation (1).

                 (Add 7 to each side)

                             (Apply additive inverse property: )

                                   (Add: )

The solution is .

The solution is .