What is the vertex of y = - x2 - 4x + 25? Is it a maximum or a minimum?

Ihave trouble finding the nth term of a linear sequence

Tim's phone service charges $26.39 plus an additional $0.21 for each text message sent per month. if tim's phone bill was $31.64, which equation could be used to find how many text messages, x, tim sent last month?

Rewrite so the subject can be y in the formula d=m-y/y+n