Question

What is the approximate measure of I

Answer 1

Let θ = m∡I

tan θ = 20/21
tan⁻¹ (20/21) = θ

θ ≈ 43.60° or if you need to round to another decimal point, plug the equation I put in bold into a calculator. 

Answer 2

Tan I = opposite/adjacent
        = segment TR/segment TI
         = 20/21
tan^-1(tan I) = tan^-1 (20/21)
                  I = 43.6°