Or, even simpler in one line for the solve, and one line each to define each of PO and PH...
PH = [101.3; 136.7; 197.3; 284.0; 456.0];
PO = [16.1; 23.1; 37.1; 57.9; 101.3];
xH = (101.32 - PO)./(PH - PO);
Learn to use vectors and arrays in MATLAB. Until then, you will find yourself writing far less efficient code, and spending far more time just writing the code.