Write double toFahrenheit(double celsius) that converts using F = C * 9/5 + 32. In main, read a Celsius value, call your function, print the result. Test with known values. 0 Celsius gives 32 Fahrenheit.
100 gives 212. If you get weird decimals, that's floating-point rounding, it's fine. This shows the pattern: take input, calculate, return result. Learn it here with a formula you can verify by hand.