12. Select three equations that represent a linear function.
a. $y=x$
b. $y=x^3$
c. $y=2x^2-4$
d. $y=1.7x+9$
e. $y=\frac{x-7}{12}$

Step 1 ：A linear function is a function that has the form of $y=mx+b$, where m and b are constants. The graph of a linear function is a straight line.

Step 2 ：We need to select the equations that fit this form.

Step 3 ：Looking at the equations, we can see that:

Step 4 ：$y=x$ is a linear function with $m=1$ and $b=0$.

Step 5 ：$y=x^3$ is not a linear function because it involves a cubic term.

Step 6 ：$y=2x^2-4$ is not a linear function because it involves a quadratic term.

Step 7 ：$y=1.7x+9$ is a linear function with $m=1.7$ and $b=9$.

Step 8 ：$y=\frac{x-7}{12}$ is a linear function with $m=\frac{1}{12}$ and $b=-\frac{7}{12}$.

Step 9 ：Therefore, the equations that represent a linear function are a, d, and e.

Step 10 ：Final Answer: $\boxed{{\displaystyle \text{a, d, e}}}$