In a triangle, the total angle is 180 degrees. A straight line is always 180 degrees. With this in mind, the following can be obtained:
[tex]a + 2x + {57}^{o} = {180}^{o} \\ 5x + a = {180}^{o} [/tex]
These are simultaneous equations.
[tex]a + 2x + {57}^{o} = 5x + a \\ 3x = {57}^{o} \\ x = {19}^{o} \\ 5x + a = {180}^{o} \\ 5( {19}^{o} ) + a = {180}^{o} \\ a = {85}^{o} [/tex]
