Answer is provided in the image attached.
-12=4e+23+e
-12=4e+e+23 - just arranging
-12=5e+23
Move +23 to the other side. Sign changes from +23 to -23.
-12-23=5e+23-23
-12-23=5e
-35=5e
Divide by 5
-35/5=5e/5
e=-7
Answer: e=-7
