HELP ME PLEASE

Arefrigeration system at your company uses temperature sensors fixed to read celsius (°c) values, but the system operators in your control room understand only the fahrenheit scale. you have been asked to make a fahrenheit (°f) label for the high temperature alarm, which is set to ring whenever the system temperature rises above –10°c. what fahrenheit value should you write on the label

Can you me find the missing length. i attached an image.

Given the two similar triangles, how do i find the missing length? if a=4, b=5, and b'=7.5 find a'