\[ y''(x)=0 \] ✓ Mathematica : cpu = 0.0031917 (sec), leaf count = 12
DSolve[Derivative[2][y][x] == 0,y[x],x]
\[\{\{y(x)\to c_2 x+c_1\}\}\] ✓ Maple : cpu = 0.005 (sec), leaf count = 9
dsolve(diff(diff(y(x),x),x)=0,y(x))
\[y \left (x \right ) = c_{1} x +c_{2}\]
Hand solution
\[ y^{\prime \prime }=0 \]
Integration twice gives
\[ y\left ( x\right ) =c_{1}x+c_{2}\]