Use (x+y)^2=x^2+2xy+y^2 to see that (10+9)^2=100+180+81=361

There are a couple different shortcuts that work here, all need to be memorized. 1) If you know the square of a number above: (X+1)^2 - (X*2)+1 = 400 - (38+1) = 361 2) If you are multiplying two numbers between 10-19 Add the second digit of the smaller number to the larger and add a 0. 19+9 = 28+0 = 280 Times the 2 second digits and add to original 9*9 = 81 81+280 = 361