Find a way to find the constant of variation, use different value for X (4)

We assume the relationship is $Y = C + kX$, where $C$ is the constant part and $k$ is the constant of variation.
Using the first condition: When $X=3$, $Y=7$.

$$7 = C + k(3) \implies C + 3k = 7 \quad (1)$$

Using your new second condition: When $X=4$, $Y=11$.

$$11 = C + k(4) \implies C + 4k = 11 \quad (2)$$

Subtract equation (1) from equation (2):

$$(C + 4k) - (C + 3k) = 11 - 7$$ $$C + 4k - C - 3k = 4$$ $$k = 4$$

Using equation (1):

$$C + 3(4) = 7$$ $$C + 12 = 7$$ $$C = 7 - 12$$ $$C = -5$$

The constant of variation is $k$.

$$k = 4$$

The constant of variation is $*4*$.