Can someone please help me, i've just been sitting here trying to figure this out. please help me

What is the rule for reflection across the y axis

which of the following sets of data would produce the largest value for an independent-measures t statistic? the two sample means are 10 and 20 with variances of 20 and 25 the two sample means are 10 and 20 with variances of 120 and 125 the two sample means are 10 and 12 with sample variances of 20 and 25 the two sample means are 10 and 12 with variances of 120 and 125

What is the result of adding the system of equations? 2x+y=4 3x-y=6