The equivalent resistance of two resistors R1 and R2 in series is R = R1 + R2. If the resistors are connected in parallel, the equivalent resistance R is related by `1/R = 1/(R1) + 1/(R2)`
Let the resistance of the two resistors that has to be determined be A and B.
`A + B = 16` and `1/6 = 1/A + 1/B`
`1/6 = 1/A + 1/B`
=> `1/6 = (A + B)/(A*B)`
Substitute `A = 16 - B`
=> `1/6 = (16 - B + B)/((16 - B)*B)`
=> `1/6 = 16/((16 - B)*B)`
=> `16B - B^2 = 96`
=> `B^2 - 16B + 96 = 0`
This equation only has complex solutions for B.
There are no two resistors which when connected in series give an equivalent resistance of 16 ohm and when connected in parallel give an equivalent resistance of 6 ohm.