Which equation represents a linear function?

�
=
1
�
y=
x
1
​


�
=
2
�
−
−
3
y=2x−−3

�
=
5
�
3
+
8
y=5x
3
+8

�
=
�
2
+
2
�
+
1
y=x
2
+2x+1