How the FUCK did he simplify the top addition to the bottom left fraction? He absolutely skipped a step. I'm not having a hard time following the actual content, but the exercises always have some unrelated bullshit like this.

I applied the formula for adding rational numbers and i still can't find any way to get from that top addition to bottom left. I can't simplify the resulting fraction because the dividend is an addition and the multipliers on the divisor don't apply to both the sides of the addition on the dividend.