how do i find angle theta using trigonometry

Use the inverse sine function on your calculator (also known as arcsin or sin¯¹)

So arcsin .125 = 7.18 degrees
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You use inverse trig        OR   arc trig      (these are different terms for the same thing)

$$\\sin\theta =0.125\\\\ then\\\\ \theta = sin^{-1}(0.125) = asin(0.125)$$

On the web2 calc it is asin