EXPLANATION: The error in the rewritten expression done by your 'friend' is that he/she did not multiply the five in the parenthesis by ALL terms (meaning numbers and/or variables) as he/she should have done according to the distributive property. In the distributive property, you distribute equally to all numbers and parts in the equation or expression.
Take, For example, 7(x+4). Instead of just multiplying 7 times x in the parenthesis, you would also multiply seven times to four and add that to the end of the expression, making the rewritten expression 7x+28.
So, the correct rewritten expression for this problem is 5x+10.
![\bf 3[cos(270^o)+i~sin(270^o)\implies 3[0+i( -1)]\implies 3-3i](https://tex.z-dn.net/?f=%5Cbf%203%5Bcos%28270%5Eo%29%2Bi~sin%28270%5Eo%29%5Cimplies%203%5B0%2Bi%28%20-1%29%5D%5Cimplies%203-3i)