Please help me on #1 and #2 i dont understand the table

Iknow the answer to 13 is b and 14 is d. i just need to know why the correct answers are correct

__can be defined as which velocity changes

Imagine that twenty i. u.’s of enzyme z were catalyzing the above reaction for one minute, under vmaxconditions, in a 3.00 ml assay volume. the assay is buffered with 20 mm phosphate buffer, ph 7.60. what will the ph be at the end of that one minute?