Beginmultline 12348x712344x35 Rightarrow x7 endmultline iiiUse emphsplit to split equations with alignment beginequation beginsplit 12348x7 12344x35 Rightarrow x7 endsplit endequation. The first part will be aligned to the left and the second part will be displayed in the next line and aligned to the right. LaTeX does allow inline maths to break over lines by default but there are a number of restrictions.
Its mostly pretty good but it sometimes gets it wrong.
Beginequation 12348x712344x35 Rightarrow x7 endequation iiUse emphmultline to split equations without alignment. One way is to use gather but I normally use align both from the amsmath package. In fact for the equations at hand theyre not needed at all. Insert a double backslash to set a point for the equation to be broken.