At what value of b does the point of intersection of lines 9x-7y = 35 and x + by = -20 belong to the axis of the ordinate?

A point on the ordinate axis has a coordinate x = 0.
Substitute this value into the system of equations and solve it with respect to the variable b.

9x – 7y = 35;
x + by = -20.

9 * 0 – 7y = 35;
0 + by = – 20.

y = – 35/7 = – 5;
b * (- 5) = – 20;
b = – 20 / (- 5) = 4.

Answer: for b = 4, the intersection point of the graphs belongs to the ordinate axis.