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Well, i can tell you one thing. Extract the unique alphabets from the given String, Get user values for those unique alphabets(whatever you call them in Mathematics, i guess variables), start...

Alright.... So, what's the problem in coding all this equation's solution? You almost did that..

Take a copy and pen, write down the steps of solving the thing. Start coding. If you get stuck, ask here.
Can you tell me by the way, how the above equation will be calculated to 258?

What have you tried so far?