The IF function consists of 3 'arguments' or parts:
=IF(CONDITION,VALUE_IF_TRUE,VALUE_IF_FALSE)
NOTE: Like with all Function, the arguments are separated with a comma (,).
=IF(ISNUMBER($W9),1+(1-$W9/$AF$7),"")
If the value/content of Cell W9 is a number the condition would be TRUE so it will proceed to the Value_If_True argument: 1+(1-$W9/$AF$7). If the value of Cell W9 is not a number the condition would be FALSE and it will proceed to the Value_If_False argument: "" (this will simply return no value (blank cell)
=IF(ISNUMBER($V9),$V9/$AE$7,"")
If the value/content of Cell V9 is a number the condition would be TRUE so it will proceed to the Value_If_True argument: $V9/$AE$7. If the value of Cell v9 is not a number, the condition would be FALSE thus proceeding to the Value_If_False argument: "" (this will simply return no value (blank cell)