According to the rule of 72, in about how many years will $78 be worth $39 if the rate of inflation is 5.8%

Your answers is 12.4 years 

One solution:
y = ax + b from our given example we get the following system:

If we subtract first equation to the second we will get the following

4h - 2h = ah + b - (3ah+b) 
2h = ah +b - 3ah - b
2h = ah - 3ah + b - b
2h = ah - 3ah
2h = ah (1 - 3) = -2ah

If we divide both sides by 2h we will get

2h/2h = -2ah/2h     which is equal to the following:

1 = -1a    from here we can assume that a = -1

so ours slope is a = -1

Second solution:
We can use the formula of slope:
slope = 

slope = 

in our case 

So we will have the following:
slope =