<h3>Algebraic expressions</h3><p>An <strong>expression</strong> combines numbers and letters, where a letter is a <strong>variable</strong>. We <strong>simplify</strong> by collecting <strong>like terms</strong>: <code>3x + 5x - 2 = 8x - 2</code>. We <strong>factorise</strong> by taking out a common factor: <code>6x + 9 = 3(2x + 3)</code>.</p><h3>Matrices</h3><p>A <strong>matrix</strong> is a block of numbers arranged in rows and columns. Its <strong>order</strong> is rows by columns. Two matrices of the same order are added by adding matching entries. Multiplying a matrix by a number (a <strong>scalar</strong>) multiplies every entry by that number.</p><p>For example <code>[2 3] + [1 4] = [3 7]</code> and <code>2 x [1 5] = [2 10]</code>.</p><div class="callout"><strong>Kenyan context:</strong> A school canteen records Monday sales as a matrix of [mandazi, tea] for two breaks: morning [40 30] and lunch [25 50]. Adding them gives total sales [65 80]. If each item later sells at double the count for a fundraiser estimate, scaling by 2 gives [130 160].</div>