12. Select three equations that represent a linear function.
a. $y=x$
b. $y=x^{3}$
c. $y=2 x^{2}-4$
d. $y=1.7 x+9$
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 ：\(y=2 x^{2}-4\) is not a linear function because it involves a quadratic term.

Step 7 ：\(y=1.7 x+9\) 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: \(\boxed{\text{a, d, e}}\)