Which characterization accurately describes both conduction and convection

Both conduction and convection need matter to transfer thermal energy.

Both only occur when currents are created by different densities.

Conduction involves collision of particles, while convection involves the movement of a liquid or gas.

or both only occur when currents are created by different densities.  

(I tried my best to find the answer, let me know if this was helpful, goodluck!)

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)