帮我解答一个英语应用题,谢谢~~

题目

帮我解答一个英语应用题,谢谢~~
Several years ago,I planted a 7-foot tree in my yard.The tree has grown 3 feet each year,and last year it was four times the original height.How tall is the tree now,and how long ago did I plant it?
最好把这到题的答案、中文意思写出来~
（是绝对鄙视,杜绝用翻译器的人的!）

答案

几年前,我在院子里种了一棵7英尺的树.这棵树每年长3英尺,去年,它是原先的4倍高了.问:现在它多高了?我种了它多久了?
设种了x年
7+3(x-1)=28
x=8
7+3*8=31(英尺)
答:现在31英尺,种了8年.