2y - ax + 4 = 0 \(\implies\) 2y = ax - 4. y + (x/4) - 7 = 0 \(\implies\) y = -x/4 + 7 Slope of line 1 = a/2 Slope of line 2 = -1/4 -1/4 x a/2 = -1 (product of the slope of two perpendicular lines = -1) a/8 = 1 \(\implies\) a = 8.