The line x - 2y + 3 = 0 has the slope-intercept form y =and has, therefore, the slope . So, it makes the angle with the horizontal axis "x" such that = . (1) The line y = mx makes the angle with the horizontal axis "x" such that = m. (2) The angle - is equal to 45 degree, according to the condition. Hence, = 1. By applying the formula for tangent of the difference of arguments, you will get then 1 = = = substitute here (1) and (2) to get = = . QED.