Answer:
The value of the unknown r is -7
Explanation:
* To solve any equation you must find the value of the unknown
- The equation has two sides equal each other
- To solve it you must separate the unknown in one side and
the number in the other side
Lets look to our problem
- The equation is 21 + 7r = -28
- We will solve to find the value of the unknown r
* Lets separate the variable
∵ 21 + 7r = -28 ⇒ subtract 21 from the both sides to put the
unknown in one side and the number in the other side
∴ 7r = -28 - 21 ⇒ add the like terms
∴ 7r = - 49 ⇒ divide both sides by 7
∴ r = -7
* The value of the unknown r is -7