Factoring trinomials in this form : 

To do this by inspection you need to find two integers which have a product of c and a sum equal to b.

Example: 

You need to identify what two numbers have a product of 20 and has a sum of 21. In many this case 20 has many possibilities for the sum and a product,, this can be a challenge. For example 4 & 5 equals to 20 as a product, however their sum is only 9 so that wouldn’t work. What would work though would be 1,20, because they make a product of 20 and have a sum of 21. Then what you need to do is factor and write out the equation

 = 

You know that this is correct when you foil the term and get the same equation.