if (x,2), (-3,-4) and (7,-5) are collinear , then x =

The given points are A(x, 2) , B(-3, -4) and C(7, -5).
The points A, B and  C are collinear if area of a triangle formed by joining these points is 0.
$$\begin{gathered} so, area of \Delta ABC = 0 \\ \Rightarrow \frac{1}{2}\left[x_1\left(y_2-y_3\right) + x_2\left(y_3-y_1\right) + x_3\left(y_1-y_2\right)\right] = 0 \\ \Rightarrow \frac{1}{2}\left[x\left(-4+5\right)-3\left(-5-2\right) +7\left(2+4\right)\right] = 0 \\ \Rightarrow \frac{1}{2}\left[x - 3\left(-7\right)+ 7\times 6\right] = 0 \\ \Rightarrow \frac{1}{2}\left[x+21+42\right] = 0 \\ \Rightarrow \frac{1}{2}\left[x+63\right] = 0 \\ \Rightarrow x+63 = 0 \\ \Rightarrow x = -63 \end{gathered}$$