y is partly constant and partly varies as the cube of x given that whenx=1,y=5,andx=2,y=-1 find the relationship betweenx and y hence find the value of y when x=3

Answer 1

Do you mean that
y = a x^3 + b ?

If not, I don't see how y can be "partly constant" etc.

If so, then use your two known (x,y) points to establish a and b.

5 = a + b
-1 = 8a + b

7a = -6
a = -6/7
b = 41/7

y = -6x/7 + 41/7