How do I input values using the combination key?

I have the TI 30XIIS and there is no NCR button...I have tried to use the PRB button by entering the first vcalue, 2nd button and scrolling over to find NCR, then entering the next value, but I get syntax error. For example for a 25 C 4 I entered 25, 2nd button, PRB button 2 times, scrolled to the right and found the nCr, pressed enter, 4, , enter...and got a syntax error...what am I doing wrong?
Help...my test is in two days! I have the TI 30XIIS and there is no NCR button...I have tried to use the PRB button by entering the first vcalue, 2nd button and scrolling over to find NCR, then entering the next value, but I get syntax error. For example for a 25 C 4 I entered 25, 2nd button, PRB button 2 times, scrolled to the right and found the nCr, pressed enter, 4, , enter...and got a syntax error...what am I doing wrong? Help...my test is in two days!