The equation is .          

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Addition property of equality: if a = b, then a c = b c.

       (Add 3.5 to each side)

                           (Additive inverse property: )

                                     (Add: 4.2 + 3.5 = 7.7)

Check:

To check the solution, substitute q = 7.7 in original equation.

  \ \

                                 (Subtract: )

Compare the values, the equation is true. The solution is q = 7.7.

The value of q = 7.7.