f'(x)=8-4x, by differentiating. When x=-2 f'(-2)=16, the slope of the tangent. We know the equation of a straight line is g(x)=mx+c, where m is the slope. So the equation of the tangent is g(x)=16x+c. To find c, substitute the point (-2,-24): -24=-32+c so c=8 and g(x), the equation of the tangent, is 16x+8. The point (-2,-24) is on the curve f(x), because f(-2)=-16-8=-24.