Actually Welcome to the Concept of the Straight lines.
Since there are more than one lines that can land plane safely, we take one of it,
When, X = 10 and Y = 7, so the equation we get as,
y-7 = m(x-10) + c
lets find slope, that is value of m,
y2-y1/x2-x1 = m
so, 9-7/ 7-10 = 2/-3
so, m= -2/3
so the final Equation is,
=> y-7 =(-2/3)(x-10) +c