You can use either
=OFFSET(F12,-1,0)+OFFSET(F12,0,-2)-OFFSET(F12,0,-1)
, or=INDIRECT("F11",true)+INDIRECT("D12",true)-INDIRECT("E12",true)
=INDIRECT("R11C6",false)+INDIRECT("R12C4",false)-INDIRECT("R12C5",false)
=INDIRECT("R[-1]",false)+INDIRECT("C[-2]",false)-INDIRECT("C[-1]",false)
Both functions also allow to specify ranges, just use whatever has your personal preference (see Excel Help)…