The cooler master 750 has four PCI-e lines, each of which carries one 6+2 pin PEG connector. Most 770's use one six pin and one 8 pin. Choose any two of the PCI-e lines, install the gpu in the uppermost pci-e lane (closest to cpu), plug in one line use both the six pin plus the two pin into the 8 pin connector, and plug just the 6 pin portion of the second line into the 6 pin connector. Let the 2pin portion of the second line "dangle" - it will be unused.
Mark