The sum of h and b is equal to 72. let h = 48. what is the value of b?

Since h = 48, we know that the equation is 48 + b = 72. We also know that if we subtract 48 from 72, we'll find b. If you don't know what I mean, here's an example.

Let's say that we had this equation,
8 + 14 = 22
Let's just take out 14 for a minute, and replace it with x. 
8 + x = 22
To find x, we have to subtract 8 from 22, which equals 14. So now we know that x = 14. 

Now back to our problem. 
72 - 48 = 24 so, 
48 + 24 = 72

Hope this helps! :)