Edit Bar controls: Apply angle from horizontal axis, Small angle, Ok
This tool adds an equal angle constraint between two pairs of lines or between a single line and the horizontal axis.
Apply angle from horizontal axis - If this option is switched on only one line needs to be identified as the first line is assumed to be the horizontal axis.
Small angle - If this option is switched on the smaller of the two possible angles is used.
To use the tool, depending on the edit bar options, identify either a single line or a pair of lines followed by the sector between the lines to specify the first angle and then the same again to specify the second angle.
Shift Modifier
You can also hold the Shift key down whilst identifying angles to include more of them. When you have finished identifying them, all of the angles will be constrained to be equal.
After identifying two angles whilst using the Shift modifier the Ok button becomes active on the edit bar: 
You can now click this button to finish the tool and apply the constraint.
After creating the equal angle constraint you can hover your cursor over the constraint and all the related entities and angles will be highlighted.
NOTE: Whilst this tool can be used to move entities so as to satisfy the constraint, it is recommended that entities are created in the desired position.