How can i get the Q operating point of the differential amplifier?
If you substitute the equation for gm in the collector resistors and make Vid=0 you get the QOP dc.

Since current gain drops rapidly to 10% max at Vce(sat) as defined in datasheets with Ic/Ib=10, for low currents keep Vce > 1V and >2V for high currents for max low swing.

For max high swing when voltage drop across collector means current has reduced in that side, this also affects current gain when it starts to get &lt; 10% of I which offers a practical distortion limit for V max unless there is high gain with external negative feedback.

Thus make any reasoned assumptions for Vce-max, min on each side , choose R to be mid scale of those limits from above with Vid=0 for a given I.

Then  each side will have max swing with reasoned low distortion from sensitivity to current gain or gm with current.

By reasoned I mean “Rule of Thumb.” 

Distortion or asymmetrical gain may be computed, but for DC operating point start with Vce min =1V and for AC use Ice min = 10%I which might also be 1V drop min across Rc thus max (fairly ) linearity is Vc max = Vcc -1 and Vce min =1.

Thus the ideal QOP, Quiescent  Operating Point = Vc on both sides. Is the midpoint of min &amp; max.