What is the solutions in maths?

In maths (or math, for Americans), a solution is a value, or a number of different values, that one can substitute for a given variable or set of variables in an equation.
Let's take the following equation as an example.

x + 9 = 15

In this equation, the solution is the value or number that we can substitute for the variable represented by the letter x. The only correct solution is the number 6, and the equation becomes 6 + 9 =15.
Here's another example:

(x + 9) ÷ y = 5.

In this slightly more complicated equation, we have two variables, represented by the letters x and y. The solution will this time consist of two values, or numbers, that we can substitute for the letters x and y. Thus, one possible solution in this instance consists of the two numbers, 16 and 5, whereby the number 16 can be substituted for the letter x, and the number 5 can be substituted for the letter y. The equation thus becomes (16 + 9) ÷ 5 = 5.