Your derivation is wrong. Use product rule

d(uv)/dx = u dv/dx + v du/dx.

the derivative comes out to be dy / dx = (1-3x)/(1-2x)^1/2

Now get dy/dx at x = -4

that is the slope of tangent at x = -4.

Negative reciprocal of this slope will give us slope of normal to the curve at x = -4.

Thus slope of normal would be -1/[dy/dx] = - (1-2x)^1/2 / (1-3x)

Now you can proceed further