Question 295406
Do this the same  as any subtraction but term by term .
<pre>
4X   - 5
3.1X + 1.8 

4.0X - 5.0
3.1X + 1.8 

 .9x - 6.8
</pre>
If you are subtracting 4x-5 from 3.1x+1.8 the answer is different
<pre>
3.1x+1.8
4x   -5
-.9x+6.8
 </pre>
You need to do something about this teacher. 
What level are you? grade school, high school, college?
Students should not be able to persuade a teacher to change answers nor methods.
There probably isn't much chance of doing anything in grade school. But if this is a middle school, high school or higher, the teacher is unqualified.