The points (−1, −2), B(7, 2) and C(k, 4), where k is a constant, are the vertices of
∆ABC. Angle ABC is a right angle
slope of line between (-1,-2) and (7,2)
= (-2-2)/(-1-7) = 1/2
slope of line between (7,2) and (k/4)
= (4-2)/(k-7) = 2/(k-7)
but they form a right angle, so
2/(k-7) = -2
2 = -2k + 14
2k = 12
k = 6
I assume you wanted to find the value of k