8:1 multiplexer to 6:1 multiplexer
Assume that D6 and D7 are always low. Trace those signals through the gates in your design. If you find gates whose output values must always be the same, those gates can be removed and their output signals changed to a direct connection to logic '1' or '0'. Repeat until no gates are removed. Then remove unnecessary direct connections to logic '1' or '0'.