This formula is an array formula. To enter an array formula, type the formula in a cell then press and hold CTRL + SHIFT simultaneously, now press Enter once. Release all keys.
The formula bar now shows the formula with a beginning and ending curly bracket telling you that you entered the formula successfully. Don't enter the curly brackets yourself.
Explaining formula in cell C11
The MID function returns a given number of characters from a value. This allows us to extract only the numbers from the value.
MID(C10,4,999) becomes MID("SEC48",4,999)
and returns "48".
This is still a text value so in order to use that, we must first convert it to a numerical value.
MID(C10,4,999)*1 becomes "48"*1 and returns 48.
The second argument in the LOOKUP function is the lookup list. This list must also be converted into numbers. I will use the same technique described in the previous step to extract and convert the numbers from cell range B3:B8.