The phrase "an extremely difficult problem" is correct and usable in written English.
You can use it when describing a problem that is particularly challenging or complex.
Example: "The researchers faced an extremely difficult problem when trying to find a solution to the environmental crisis."
Alternatives: "a highly challenging issue" or "a very tough dilemma."
Georges, for his part, seemed worried that Simons was overly focussed on an extremely difficult problem.
"We've concluded that it's an extremely difficult problem," said a Pentagon antimissile scientist.
"It's an extremely difficult problem," said Yoshio Ando, an official at Reconstruction Headquarters.
To be without Joe Launchbury, Geoff Parling and Courtney Lawes is an extremely difficult problem to get over.
Control of a scale model autonomous helicopter during takeoff and landing manoeuvres has proved to be an extremely difficult problem.
She said it was an extremely difficult problem to legislate against but political parties could perhaps work harder to stamp out intimidation among their supporters.
The deep phylogeny of eukaryotes is an important but extremely difficult problem of evolutionary biology.
Accurate estimation of the divergence time of the extant eukaryotes is a fundamentally important but extremely difficult problem owing primarily to gross violations of the molecular clock at long evolutionary distances and the lack of appropriate calibration points close to the date of interest.
In this research, a novel hybrid thermodynamic cycle with an asynchronous buffering heat recovery process is proposed to overcome the extremely difficult problem for micro swing engine, i.e. the far too low efficiency.
One way to solve the extremely difficult problem of forecasting solar deployment is the logistic function or S-curve - a simple method that enables learning by interpolating between past exponential growth during PV's early adoption phase and its eventual saturation.
