Most of Econ 101, as a sibling comment notes, from basic approaches (definitions, deductive reasoning from models using algebra, testing of models with data) to detail (marginalism, "new consensus" macro modelling).

Macroeconomics and econometrics are two major subfields which literally didn't exist in Marx's day: economists may disagree significantly on conclusions and somewhat less significantly on certain assumptions models make, but they're focusing on the same variables (interest rates matter and fiscal policy somewhat matters, inflation is a thing which can be reduced, boosting employment and staving of recessions is more difficult) and doing similar maths.