Because variable c and variable d are not values but strings(texts) you couldn't do it directly..
To calculate the value variable f
You should first check the variable d:
System compare variable d = "+"
- set variable f = variable a + variable b
System compare variable d = "-"
- set variable f = variable a - variable b
System compare variable d = "x"
- set variable f = variable a * variable b
System compare variable d = ":"
- set variable f = variable a / variable b