The following is the generic formula to extract the first name from a full name.
=LEFT("Full Name",SEARCH(" ","Full Name")-1)
SEARCH function in the formula will return the position of ” ” (Space character) from the Full name.
In the formula used in the cell C3, SEARCH function will return the position of ” ” (Space character) in the name Christopher Nolan, which is 12.
After subtracting 1 from this value, the result is used in LEFT function to return the characters to the left from this position.
Another version of the same formula
SEARCH function in the above formula can be replaced with FIND function.
=LEFT("Full Name",FIND(" ","Full Name")-1)