。
She rested her elbows on both knees.
Literal
She [topic-は] both-knees [on-に] elbows [object-を] placed.
両膝 (りょうひざ, 'both knees') uses the prefix 両~ meaning 'both.' のせた (載せた, 'placed on top') describes putting something on a surface. Body-part vocabulary: 膝 ('knee') and 肘 ('elbow') are useful to know.