The easiest way to do this is to realize this is a weighted average of the two points. (A weighted average is a linear combination where the coefficients add to one.)
(x,y) = (1-t)C + tD
where t is a real parameter. When t=0 we're at point C, when t=1 we're at point D. We're interested in t=1/4,
(x,y)= (3/4) C + (1/4) D
We're only asked for the x coordinate:
x = (3/4) (1) + (1/4) (-4) = 3/4 - 1 = - 1/4
Answer: [tex] \quad - \dfrac 1 4 [/tex]
