back up: 支持，为…撑腰
But he's got science to back up the results.
但他得到了科学依据来支持这个结论。
Senior administration officials back up the president.
美国政府高级官员们都支持总统。

hang up: 把…挂起
He says when I hang up.
我挂上电话后他说道。
Hang up these dresses.
把这些连衣裙挂起来。

cross the line“跨过线”，可以理解为“做的太过分了
Few would cross the line to shoplifting, which they ( correctly) regard as criminal.
少数人会超越底线去偷窃，他们（正确地）认为这是犯罪。
For some, revenue projections cross the line.
就某些IPO公司来说，透露收入预期也是违规行为。