Sometimes we call it variable or unknown.
In x+2=9, x is variable but we can work out if we try!
A variable does not have to be ‘ x ‘, it may be ‘ y ‘, ‘ z ‘ ,’ w ‘ or any letter named or symbol.
The above question could be asked what does x means in math.
Examples:1 what is x
Find the value of x, if x+9=4.
subtracting ‘9’ on both sides