Op. amp. with integrator - enough values to start?
Not right. You should deduce Uref from the output waveform. It's well possible after you have made it clear how the circuit works in details

Some guidance:

Find how to make the switching levels of the schmitt-trigger to be the extremes of the triangle waveform. You have 2 unknowns (one voltage; one resistor) and 2 voltage division equations.

Then find an integration time constant which makes 15V charging in 2ms with 15V input to be true. You can select integrator's R or C freely, if the parts are ideal. In practice you must obey output current limits and the need to keep the drift low enough.

ADD due the comments. Seemingly you have skipped some basic lessons. Now it's the right time work them.

Time constant: The output of the integrator changes just amount -U in time RC, where U=the input voltage of the integrator. You need -15V transition in time 2ms when the input is 15V from U13. RC=2ms is right. 1uF &amp; 2kOhm are ok. Output current 7,5mA isn't impossible for practical opamps altough 0,1uF and 20kOhm would need only 10% of the current. 

NOTE: Your Schematic possibly demands that R31=R36. Then you cannot select R31 freely, you must use the value which satisfies the Schmitt trigger's switching levels.

The Schmitt trigger (=ST) should flip its output when the integrator output reaches +10V or decays to -5V. In the former case ST's output is -15V until the +10V limit is reached and after it ST's output is +15V until the -5V limit is reached.

See this generalized voltage divider:



You must be able to prove with elementary DC calculations the formula of Ux in case Ux is not loaded with any current. If you cannot prove this when your teacher wants, you will fail.

Let Ua be the output of the integrator, Ux=the voltage in the plus input of U13 and Ub = the output voltage of U13. We can use Ux's formula if we assume U13 is ideal (= its inputs do not take significant currents and the output really reaches plus and minus 15V)

Now you should write Ux=Uref in two cases: 1) Integrator's output reaches +10V but the ST still outputs -15V.  2) Integrator's output has dropped to -5V but the ST still outputs +15V.

Now you have 2 equations which give Uref and R36.