Start with: Let m∠S be S

Then ask yourself, what is the measure of ∠T?

Next thing to ask yourself is: What is meant by the angles are SUPPLEMENTARY?

Research this or use your textbook or the internet and you should find answers to these questions.

When you do, you should be able to form a LINEAR equation, in 1 variable, and then solve for that variable. 

Then use the value for that variable to find the value for the other variable. Thus, you'll have m∠T.

OR

You could also start with: Let m∠T be T

Then form your equation and continue to solve for m∠T. 

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