3-3×6+2

Multiplications are done before additions or subtractions,
so do 3×6 and replace 3×6 by 18 and copy everything else over:

3-18+2

All that's left is a subtraction and an addition.  Those are done
left to right. The left-most is 3-18, so do 3-18 and replace 3-18
by -15 and copy over the rest:

-15+2

Now there is just one thing to do, the addition of -15 and +2,
which gives the final answer:

-13

Edwin