Chapter 1 - Linear Equations in Linear Algebra - 1.3 Exercises - Page 33: 18

h = -7/2

In order for y to be in the plane, the linear system has to be consistent (i.e. there has to be a solution). Writing the linear system in the form of a matrix [a1 a2 | y] and finding the matrix in Reduced Echelon Form will give the last row [0 0 | 7 + 2h]. For the system to be consistent, 7+2h will have to equal zero, hence h= -7/2