The equation is 18 = 5p + 3           

According to Subtraction property of equality: if a = b, then a c = b c.

18 3 = 5p + 3 3               (Subtract 3 from both sides).

  18 3 = 5p                                      (Additive inverse property: 3 3 = 0)

  15 = 5p                                    (Subtract: 18 3 = 15)

\ \

According to division property of equality:

For any number a, b and c, If a = b then  \ \

                                (Divide both sides by 5)

                                    (Cancel common terms)

p = 3                                        (Divide: )

Check:

To check the solution, substitute p = 3 in original equation.     

18 = 5(3) + 3

15 = 15 + 3                              (Multiply: 5(3) = 15) \ \

18 = 18                                    (Add: 15 + 3 = 18) \ \

Compare the values, the equation is true.

The value of p