Find your starting word for Part. 3

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FHKOPGTESVQA FIZQTOKEEHOU MOCVRRPAVXLW WWYWHWPNEWGX DTTDLXQCNZDN VISHQEPPPXCW DUFTIDVZUDCP YAGMMRMLZJSJ NPDOWNTXJOTM CXKTLCJYLKQM QZDFENVZXKXX KLZARFSAPILC
1.
simplified version of C to F is: Multiply by 2 and add __?
2.
largest prime number under 10
3.
24 options for this in the crossword