Evaluate the expressions then , place the expressions on the corresponding locations on the number line

3.4a=57.8 what is the the value of a

Grandpa ernie is shrinking! over the past 4 years his height decreased by a total of 2.4 cm. it decreased by the same amount each year. what was the change in grandpa ernie's height each year

Find the value of x. also give me an explanation so i can complete it to other questions as well, : )