where

Pt = power transmitted by the radar (watts) <- it is constatnt at this case
Gt = gain of the radar transmit antenna (dimensionless) <- it is constatnt at this case
R = distance from the radar to the target (meters)
S = radar cross section of the target (meters squared)
Aeff = effective area of the radar receiving antenna (meters squared) <- it is constatnt at this case
Pr = power received back from the target by the radar (watts) <- it is constatnt at this case

moving all the constants to the left...

( Pr * 4^2 * PI^2 ) / ( Pt * Gt * Aeff ) = S / ( R^4 )

Const = SMiG / RMiG^4
Const = SF117 / RF117^4

as the constants are the same with both targets, the two equations can be made equal...

SF117 / RF117^4 = SMiG / RMiG^4

...