Looks like distributive property 5 times 5 is 25, 5 times a -5i is -25i , 5i times 5 is positive 25i , 5i times 5i gives you , 25 i squared , so now you combine like terms , you have 25 -25i + 25i - 25i squared , cross out -25i and the positive 25i anf your left with 25-25 squared .