In school we started doing algebra but I don't know where to begin with , can someone help me please?
Thanks
5x=15
If it's a simple solve for x, it would be 15/5=3
Some textbooks might be useful for homework help. If your school has prescribed some, just use those. Frequently getting some help from a senior might also help. Asking your teacher to explain again, should not be an issue. Most teachers are happy to help.
the key rule in algebra is anything you do to one side of the equation you must do to the other side of the equation.
$5x = 15$
here we would like to isolate the $x$ on one side of the equation.
Let's multiply both sides by $\dfrac 1 5$
$\dfrac 1 5 \cdot 5 x = \dfrac 1 5 \cdot 15$
$x = 3$
and voila!
Think of it as "something has been done to x and to get back to x, we have to 'undo' it". And "undoing" a mathematical operation means doing the opposite (technically the "inverse"). In 5x= 15, x has been multiplied by 5. The "opposite" of "multiply by 5" is "divide by 5" (or equivalently, as romsek said, "multiply by 1/5"). And, of course, to keep the two sides equal, whatever you do to one side of an equation you have to do to the other.