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Trying to get a program to fine the slope and intercept of a line on a graph....

I've got it all working so far...except the last line. I broke it down into pieces and ran it assigning numbers to the variables, but in the program, it doesn't work. Here is the code...

```
def slope(x1, y1, x2, y2):
a = (float(y2 - y1))
b = (float(x2 - x1))
m = float(a / b)
def intercept(x1, y1, x2, y2):
x = x1
y = y1
m = slope(x1, y1, x2, y2)
b = y - m*x
return b
```

It keeps giving me this error

```
Traceback (most recent call last):
File "<pyshell#4>", line 1, in <module>
intercept(12, 20, 24, 40)
File "C:\Users\Tony\Documents\Module 2 SLP\slope", line 11, in intercept
b = y - m*x
TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'
```

If I'm understanding this correctly, it's telling me the * is an unsupported operand...but I thought * was an operator?

I tried

`b = y - (m*x)`

and that didn't work either....any hints?