How to align sketch with the x-axis
It's rather simple:


Start to edit the sketch with this line via a right-click edit sketch
click the line you want to force horizontal
on the right menu, the lower menu has "align horitontal/vertical", click it
DONE


To find the button I mean, a picture with the right tool next to the red mark:



Hint: in the case of your item, I suggest to use the blue marked symmetry on the two lines you forced to be equal:


first choose the lines (SHIFT)
choose the tool
click the symmetry line (in your case: the construction line)