Watch these videos and take notes. A formula is an equation that expresses a relationship between two or more quantities. When one of these quantities needs to be rewritten in terms of others, the formula becomes a literal equation.
In the first video, the formula for the perimeter of a rectangle (equation used to find a perimeter when its length and width are known) is solved for the width. That is, the result is an equation used to find the width of a rectangle, when its perimeter and length are known. Note that Sal Khan shows two different ways to arrive at the answer.
The formula shown in the second video converts Fahrenheit temperature into Celsius. Solving for Fahrenheit temperature results in a formula that converts Celsius temperature into Fahrenheit. Note that there are two possible approaches to do this, but only one is shown in the video. You might want to try the second approach (distribute 5/9 over the parenthesis) to see which one is more convenient.