Question 243423
Let A=Angle S=Supplement

Key point to remember is that the an angle plus its supplement always equals 180 degrees.

180 = A+S (1)
58 = A-S
58+S = A 
Subsitute 58+S into equations (1)
180 = 58+S+S
180 = 58+2S
122 = 2S
61 = S
119 = A

Proof 119-61=58