can i connect wires in a single transformer to get higher volts?
If you connect to the two 15 volt terminals, you will get 30 volts.

Between the two 12 volt terminals, you will get 24 volts.

Between the left 15 volt terminal and the right 12 volt terminal you will probably get 27 volts (but may only get 3 volts, depending on how the terminals are arranged relative to the secondary winding taps).

The 0 volt terminal will be the centre tap of the secondary winding, and the two 15 volt terminals will be the two ends of that winding.  The 12 volt terminals will be connections 20% in from the ends of the windings