arithmetic
posted by Vivian .
The first three terms in an arithmetic are given by x,4x+5 and 10x5.find the value of x.

Do you mean an arithmetic SERIES?
(With the accent in the syllable MET?) In that case, "arithmetic" is an adjective with a specific meaning. There are many other kinds of series.
If so, the difference between consecutive terms must be constant.
That means that 3x + 5 = 6x  10
3x = 15
x = 5
The first three terms would be
5, 25 and 45.