Question: I want to make a program that can calculate physics equation where the user enters different parameters: I know its simple like:

```
v=5
t=7
s=v*t
print(s)

```

its only calculating `s = v*t`; however, when I want the equation of `v` if show me an error. Hard coding `v = s/t` gives me the correct result:

```
s=5
t=7
v=s/t
print(v)

```

I want an equation that can solve with different user input; that is if user inputs the `v` and `t` , the equation will return `s = v*t` and if the user inputs `s` and `t` the equation will return `v = s/v`.

Answer: 
I'm not sure if this is what you want, but you could define three separate functions, one for each variable. For example,

```
def s(v, t):
    return v*t

def v(s, t):
    return s/t

def t(s, v):
    return s/v

```
```
def h2(v,d,t):
    title_ar = 'السرعة'
    title_en = 'speed'
    if v=='':
        print("v= ","%.2f"% (d/t),"m/s")
    elif d=='':
        print("d= ","%.2f"% (v*t),"m")
    elif t=='':
        print("t= ","%.2f"% (d/v),"s")
    else:
        print("please write all data.")

h2('', 2,6)
h2(70,'',8)
h2(25,3,'')

```