Why can you subtract an even number from another even number?

Tina practice piano for 15 hours last month and 45 hours this month use multiplication to write a statement comparing me i was tina practice during the two months use addition to write a statement comparing the hours tina practice during the two months

Will give brainliest sum of arithmetic series (sigma notation) find the numerical answer to the summation given below. (image shown below)

For f(x) = 3x+1 and g(x) = x2 - 6, find (g/f)(x)