Hint: start by asking what quantities you don't know and give them names. So you don't know "how many simple calculators" were sold and "how many scientific calculators" were sold. Give those quantities names, say $x$ is the number of simple calculators sold, and $y$ is the number of scientific calculators sold. Now, try to write the facts that you have as equations. The store sold 35 calculators total... so $x+y=35$. Each simple calculator sells for \$5, so the amount of money collected for selling simple calculators is $5x$. \$16 is collected for each scientific calculator sold, so the total from selling scientific calculators is $16y$. All together you collected \$340. This gives you an equation $5x+16y=340$.
Now, what is left is to solve two equations: