Mark's height last year was 46 inches.
Mark definitely grows over the past year. Let the height he grew be x.
Then, Mark's new height will be
[tex](46+x)\text{ inches}[/tex]
Let us represent Mark's height with M and Peter's height with P.
This means that
[tex]M=46+x\text{ -----------(a)}[/tex]
and, from the question, Peter's height is
[tex]P=51\text{ ----------(b)}[/tex]
The question says that Mark's height is 3 inches less than Peter's height. This we can write as
[tex]M=P-3\text{ -------------(c)}[/tex]
Therefore, if we put Mark's and Peter's ages into equation c, we can find a value for x as follows:
[tex]\begin{gathered} 46+x=51-3 \\ 46+x=48 \end{gathered}[/tex]
Since 46 + x = M, then Mark's height is 48 inches
