Our great sponsors

ExprTK
C++ Mathematical Expression Parsing And Evaluation Library https://www.partow.net/programming/exprtk/index.html
It seems like you mean evaluate not solve. In that case check out exprtk (github).
NOTE:
The number of mentions on this list indicates mentions on common posts plus user suggested alternatives.
Hence, a higher number means a more popular project.
Related posts
 Better library for evaluating math expressions stored on a String
 What do I do if a repo only has a make file and I'm on windows?
 Regular language input math operations
 How to calculate an expression that is stored in a string in C++
 A boolean logic simulator written in c++ & opengl 3.3. This is a toy project I'm currently developing to help me learn c++ & opengl. No libraries used, apart from stb_image and glad.