'Ten minus a number' this translates as:
[tex]10-x[/tex]'is greater than or equal to 21'. This means that the expression above is > or = to 21:
[tex]10-x\ge21[/tex]To solve it we have to do the same operations in both sides of the inequality.
First we add x in both sides. This way, we'll get possitive x on the right side:
[tex]\begin{gathered} 10-x+x\ge21+x \\ 10\ge21+x \end{gathered}[/tex]And finally we substract 21 on both sides:
[tex]\begin{gathered} 10-21\ge21+x-21 \\ 10-21\ge x \end{gathered}[/tex]And finally we compute the substraction. The solution of the inequality is:
[tex]-11\ge x[/tex]That can also be writen as:
[tex]x\leq-11[/tex]Is the same expression written backwards, it's easier to read.