Observe that

In the original equation, divide both sides by
:


Next,

where
is any integer. Then

Fix
to ensure
, so that

Answer:
r = 7
Step-by-step explanation:
Given
r + 15 = 4r - 6 ( subtract 4r from both sides )
- 3r + 15 = - 6 ( subtract 15 from both sides )
- 3r = - 21 ( divide both sides by - 3 )
r = 7