Adding and multiplying integers
Adding a negative moves left along the number line; two negatives multiplied together give a positive.
A worked example
For -11 + 7, start at -11 and move 7 steps right, ending at -4. Multiplying, -8 x -4 = 32, since two negatives make a positive.
The mistake people actually make
A larger digit after a minus sign means a smaller number: -20 is less than -6. Subtracting a bigger number from a smaller one lands below zero.