Let's factorise it :
[tex]\: {\qquad \dashrightarrow \sf ( 10 - y ) ( 10 + y) }[/tex]
[tex]\: {\qquad \dashrightarrow \sf 10(10 + y) + [ - y(10 + y)] }[/tex]
Using distributive property we get :
[tex]\: {\qquad \dashrightarrow \sf {10}^{2} + 10y+ (- 10y - {y}^{2} ) }[/tex]
[tex]\: {\qquad \dashrightarrow \sf {10}^{2} \: \cancel { + 10y }\: \: \cancel{ - 10y } - {y}^{2} }[/tex]
[tex]\: {\qquad \dashrightarrow \sf 100 - {y}^{2} }[/tex]
Therefore,
[tex] {{{\qquad \dashrightarrow \sf ( 10 - y ) ( 10 + y) = 100 - {y}^{2}}}}[/tex]
