formulas - finding what is "x"?

[Van]

Its been a while since i was at school - dont know if this relates to
"equations" but, which formula would i use if i wanted to calculate what the
value of "x" is? For example:

4 + x = 7

so the answer would be x = 3

Any ideas anyone???

 

[Teethless mama]

x=7-4
x=3

let's say a1: holds 4 and b1: holds 7
In C1: =b1-a1