Hmmm... I'm not sure where's the missing point, but i get the result : Vref=0,5Volts and R2=2,33kOhms.
I explain
Vcc=+15Volts, Vee=-15Volts
Vref=?
R1=?
R2=?
R3=10KOhms
Vutp=4Volts
Vltp=-3Volts
Vutp= Vref*[R3/(R2+R3)]+Vcc*[R2/(R2+R3)]=Vref*[10k/(R2+10k)]+15[R2/(R2+10k)] <=> Vref= {[4-15*[R2/(R2+10k)]}/10k]}*[R2/(R2+10k)] (1)
Vltp=Vref*[R3/(R2+R3)]+Vee*[R2/(R2+R3)]=(1)={[4-15*[R2/(R2+10k)]}/10k]}*[R2/(R2+10k)]*[R3/(R2+R3)]-15*[R2/(R2+R3)] <=> R2=2,33kOhms (2)
Replacing R2 with the (2) result in equation (1) i get : Vref=0,5Volts