This is the concept of algebra, we are required to write the function representing the expression given.
After the the numbers were substituted we got:
1.
f(32)=120+5(32-30)
given that 120, 5 and 30 are not changing in all the consecutive substitutions, it implies that they are constant.
Therefore the rest are variable, hence our expression can be written as:
f(x)=120+5(x-30)
The above expression is a straight line equation, representing a linear expression between variables.